I usually use a clicker to stop barking like that. Go outside with the dog. When he barks, say "quiet" in a calm voice. Wait for 5 seconds of silence, click, then treat. When you can reliably get 5 seconds of silence, move up to 10 seconds. WHen that is reliable move to 15, 20, 30, 45, 1 minute, etc. WHen you get to a few minutes, the problem is usually solved.
The dog is probably barking because something outside is scaring him or he is just nervous about the outside in general. Either way, he obviously was never taken out much in his first 3 months of life. I would put him on a leash and take him for a walk every day. Go to different places, meet people, other dogs, etc. Show him birds, squirrels, busses, cars, trucks, etc.
If he is just absolutely terrified on these walks, you need to take them slow but working up to it. At first just walk to the edge of the yard, then down the street 15 or 20 feet, the 50 feet, then 100. Whatever he can stand without freaking out.
Make the walks fun. Be upbeat and energetic. Have a good time yourself. Enjoy your time together on the walks. If he does become terrified on walks, don't coddle him or baby him. Just take him back inside.
The combination of these two things should take care of the problem but remember dogs bark. Don't expect him to never bark again. Just expect him not to be afraid of outside and to stop barking when he is told.
I hope all this makes sense. I could explain it a lot better in person.