I thought I answered this already, but I think I must have posted to something very similar. Leash walking and being outside are two different things. I would train him to walk on a leash in an environment he considers safe and separate that from learning outside is a good place. By doing both at once, you're asking a lot out of that baby brain.
Leash Walking:
I am not of the school of dragging a dog on a leash as I've seen too much damage, both physically and psychologically, done to dogs. Also, in the wild, wolves are not tied together by the neck (sorry, I just have a real problem with this). Instead, put the collar or harness (harnesses are now being recommended for walking by many trainers and vets as the collar not only puts a lot of pressure on the neck, it also lies on a pressure point that produces a sharp pain and has been attributed to the development of aggression) and leash on puppy in an area he feels safe. Use some type of lure (food, a target stick, etc.) to get puppy to move. I personally love using the target stick as my back gets sore with little dogs like Bostons

Every time puppy moves, give rewards (treats, praise, petting, etc). Start to raise criteria (first rewards for one step, then puppy needs to take two steps, then three, etc). If you raise criteria too quickly, you'll notice the puppy isn't getting it and not moving. This isn't because he is stubborn or slow, you've just asked more than he can understand. Think of it as learning a new language. If you learn two words and then someone asks you a question in the new language, you don't understand what they are asking of you.
New Environment:
I'm a firm believer in making the new place good. Lots and lots of rewards in the new place. Bring familiar things with you. Let the puppy hide behind you if he wants. If puppy ventures out to something strange, call him back and reward him. The idea here is that if he is "brave," he gets rewarded both through treats and going back to the safe zone (you). I find that soon the puppy realizes things are not so scary.
A few things to consider are the facts that, depending on his age, he may be going through a fear period. How old is he? Also, please keep in mind you haven't had him long. In essence, you are a stranger to him. I'm not saying he isn't affectionate or loyal. Rather that you haven't built up your bond yet and he may not always trust you yet. Be patient; he will
Good luck!