Take a deep breath! 
I agree with Shakespaw, that sometimes it takes time for personalities to come out. When pups are nervous being in their new homes, they can be a bit reserved.
Also, if you are nervous about things, your dog will pick up on that nervousness, and may feel he has to "protect" himself and you.
A quick bit of info on my situation, in the hopes that it will make you feel better. When I got my first puppy, Harper, I was a first time dog owner, and he was 8 weeks old. He (unlike your dog) was soooo tiny that I was nervous about walking him, especially when we came across other dogs. I am pretty sure he picked up on that. He became pretty reactive around some people, and all dogs, partly due to my nervousness, but, maybe, partly just due to his personality.
I was SO sad. I had been looking forward to taking him hiking, camping, on doggy play dates, all the fun stuff you can do with dogs, but he barked and pulled on the leash and growled and lunged. I felt like a failure, and was bummed.
I got lots of advice here on helping with his reactive issues. I was told to AVOID other dogs at first. I couldn't figure that out. I didn't want to AVOID the problem, I wanted to fix it. BUT, what I didn't realize was that these "reactions" he had caused stress hormones to go crazy in him, and just as with humans, when you have shocks, or things that really upset you, it can take a long time to calm down. I was told it could take days and weeks for the stress hormones to subside for Harper, after having a reactive episode.
So, by avoiding his triggers, I would let him calm down. I walked him at odd hours, or in areas that I was pretty sure I wouldn't come across people or dogs (even if that meant driving to a different area). Very gradually, I started to do walks where we might actually see other people/dogs. When we did, I would back up or cross the street, or go down a side street, or whatever so that he could see the dog/person, but at a distance. Whatever distance worked (he could watch, but didn't react) was his "threshold". Gradually, the goal is to shrink his "threshold".
Now, he can walk by 95% of people without a reaction. Some people he just doesn't seem to like. Funny thing, the people he doesn't like are people I feel uneasy about too! And, we still cross the street when we see other dogs, but we don't have to get nearly as far away as before.
Bottom line, and what I've learned: every dog is different, just as with people, some dogs don't do well in every situation. With some dogs, you can correct issues, with some dogs you can only manage issues. Harper is manageable. But, he is who he is, and we love him to pieces, as he's a total cuddle bug and very sweet with us and people and dogs he knows well. So, we accept the dog he is, and we work to manage his environment. But, it is, in no way, as horrible as I envisioned when I realized his issues.
Sorry, turned out to be more then a quick bit of info, but I really felt for you, and wanted to share, it gets better!
I agree with Shakespaw, that sometimes it takes time for personalities to come out. When pups are nervous being in their new homes, they can be a bit reserved.
Also, if you are nervous about things, your dog will pick up on that nervousness, and may feel he has to "protect" himself and you.
A quick bit of info on my situation, in the hopes that it will make you feel better. When I got my first puppy, Harper, I was a first time dog owner, and he was 8 weeks old. He (unlike your dog) was soooo tiny that I was nervous about walking him, especially when we came across other dogs. I am pretty sure he picked up on that. He became pretty reactive around some people, and all dogs, partly due to my nervousness, but, maybe, partly just due to his personality.
I was SO sad. I had been looking forward to taking him hiking, camping, on doggy play dates, all the fun stuff you can do with dogs, but he barked and pulled on the leash and growled and lunged. I felt like a failure, and was bummed.
I got lots of advice here on helping with his reactive issues. I was told to AVOID other dogs at first. I couldn't figure that out. I didn't want to AVOID the problem, I wanted to fix it. BUT, what I didn't realize was that these "reactions" he had caused stress hormones to go crazy in him, and just as with humans, when you have shocks, or things that really upset you, it can take a long time to calm down. I was told it could take days and weeks for the stress hormones to subside for Harper, after having a reactive episode.
So, by avoiding his triggers, I would let him calm down. I walked him at odd hours, or in areas that I was pretty sure I wouldn't come across people or dogs (even if that meant driving to a different area). Very gradually, I started to do walks where we might actually see other people/dogs. When we did, I would back up or cross the street, or go down a side street, or whatever so that he could see the dog/person, but at a distance. Whatever distance worked (he could watch, but didn't react) was his "threshold". Gradually, the goal is to shrink his "threshold".
Now, he can walk by 95% of people without a reaction. Some people he just doesn't seem to like. Funny thing, the people he doesn't like are people I feel uneasy about too! And, we still cross the street when we see other dogs, but we don't have to get nearly as far away as before.
Bottom line, and what I've learned: every dog is different, just as with people, some dogs don't do well in every situation. With some dogs, you can correct issues, with some dogs you can only manage issues. Harper is manageable. But, he is who he is, and we love him to pieces, as he's a total cuddle bug and very sweet with us and people and dogs he knows well. So, we accept the dog he is, and we work to manage his environment. But, it is, in no way, as horrible as I envisioned when I realized his issues.
Sorry, turned out to be more then a quick bit of info, but I really felt for you, and wanted to share, it gets better!
