Help! My dog will not finish his food

What can I do to get my dog to eat more? For most of his life he has never been a big eater. Since he is my first dog I've had, when he was a puppy I used to give him a lot of human food or table scraps. I regret ever doing that because I think I made him into the picky eater that he is now. All of my other dogs eat fine and they pretty much eat whatever I give them.

My dog Buddy is a miniature poodle, weighs approximately 15lbs, and is 8 years old. He has always been thin and the vet annual exams always come out fine. For his first 2 or 3 years I fed him Eukanuba small breed dry food. Since I was ignorant and didn't know nothing about nutrition around his 4th year I switched him to Pedigree small breed. These past 6 months I've been taking him to the vet because of back problems and possibly joint problems too. Because of all this I've recently decided to change his food and I am in the process of experimenting with different brands.

The first one I tried was Innova small bites. I think my dog loved it (btw the food had a similar fishlike smell to Eukanuba) but I took him off it because I found out that P&G bought out Natura products. I then switched him to Blue Buffalo Wilderness small breed which he hated. He barely ate it when I softened the food w/water. He will not eat the little black pieces. After a few days he went a day without eating so the next morning he vomited yellow bile. I found that this only happens when a dog is starving. I immediately stopped BB and switched him to Orijen. The food looks so good on paper, it even smells good haha, but he got bored of it. Every meal he started eating less and less. Now I have him on Solid Gold Wee Small bits. Same problem, he will finish his meal the first 2 days after that he will not finish the food.

For all of these high quality foods I've noticed that most of their feeding guides say that he is supposed to eat about 1 cup a day. After a few days of eating the new food he will eat about 3/4 cup. Yesterday he only ate about 2/3 cups. I can force him to finish it but only works for a couple of days. I force him by bringing my other dog and pretend to give it to her. He'll then come back to his plate, growl at her, take a few more bites, and then leave. Eventually he'll stop getting jealous and just let my other dog finish his food. Someone please give me some advice, I don't know what to do.

I've recently begun to change his feeding style from free feeding to scheduled. He has now been on a feeding schedule for about a month and I've managed to get him to eat when I put his plate down but the problem is that he won't finish his meal.

This feeding schedule has made my other dog Dixie into a wolf eater, she goobles her food down and she never did that when I free fed them. Is it possible that a dog is picky for life? I have not given my dog any treats for 2 months now. This one time I left a sandwich on the table, he jumped on the dinner table and ate it. That was about a month ago and ever since I am very careful about leaving food outside. Nobody on my family has given him humman food or table scraps. I take him out every morning for a 15-30 min walk and in the evening for another 15-30 walk.

I can feel my dog's ribs and bones but I don't think he is anorexic. He barks and runs at cats that pass by my window, he jumps up and down when I'm getting his leash, and basically drags me when I take him out for walks (yes he is on a harshness leash). He has had some loose stools because of all the food changes but on this Solid Gold his stool seems to be improving day by day.

I just wish that he would fisnish his meals because I've heard that beeing too thin can be bad for the dog's bones and joints.

Help me out folks. How can I get him to eat? He almost makes me think that he really cares about what he looks like, as if he refuses to gain any weight. Seeing him get a little fatter would be a dream come true.

I really don't wanna try wet food or raw becaue I don't have the time and money to feed him that type of diet. I have 5 dogs and he is the only one that is so fussy. I care about him a lot because he is my 1st pet ever and he is the oldest too.

I am sorry about this very long post but as you'll can see I am not a man of short words.

PS: Buddy already had heartworm test and fecal exam on February. A month ago when he had neck problems he also had blood tests for kidney and other organs and a pancreatitits test. He even had Xrays of his chest and abdomen. All came out negative. He had his teeth cleaned by the vet like 3 months ago. He is on heartworm prevention too.