Hi Vared,
Your dog needs to be properly socialized to dogs. The important point to keep in mind is that the critical socialization period for puppies extends until approximately 16-18 weeks of age. After that it is much more difficult to socialize puppies. So, you should actively socialize him daily.
You should do this with anything that he is going to come into contact with for the rest of his life. Of course this includes dogs, sounds, people, locations, etc.
There are two blog posts that might help out.
Socialize Your Puppy Now
Don't Wait Until Spring to Socialize
With dogs (or anything that you want to socialize him to) you should bring FANTASTIC treats with you and AS SOON as he sees a dog, say, "Yes" or make another noise and give him a treat. You want to use his favorite treat, because if he doesn't take it, it is a sure indication that he is stressed out. If this occurs, move him farther away from the other dog until he is taking treats.
Do this with every dog that he sees, even if he currently likes them. You want him to LOVE every dog, not just tolerate them.
30 minutes is not too long to walk a puppy unless he is over-stimulated or tired. Let him tell you if he is anxious or tired. If so, don't push him, let him be comfortable or take him home.
Let me know if you have more questions.