I've tried a couple of times to take my pitlab for a walk on a leash, but he will only walk with me if there is a treat in it for him. Other wise, he finds the nearest lawn and lays down. I know he isn't tired, since he was just playing in the backyard with his sister. How old do puppies usually get when they want to go for walks? Should I be trying something else to motivate him? I don't want him getting fat

Any help is appreciated.
Thanks!
Puppies do not need to be taught to walk, run, etc. It is part of their genetic program to move that way.
However, walking while attached to a lead is
not part of their genetic program.
Puppies have to be taught - i.e., trained - to walk on a lead.
It's best to start that training as soon as you get your puppy home.
There is no magic age when a dog will just start walking on a lead. If you don't teach it, they won't do it. I have experienced many adult dogs who did not know how to walk on a lead because they were strays or because their owner(s) didn't teach them. It's much harder to teach an adult , but it can be done - it is easy to teach a puppy.
There are a myriad of ways to teach this. Here is just one:
Leash Training for Puppies
Positive training depends on rewarding desired behavior and food is the usual reward. If you keep your training treats small - about half a dime in size - you won't be overfeeding.