Leash training is very important. Puppies are generally always great off leash at first, then they get confident and become less great. You have to train their recall. But, GSDs are big dogs. They're a breed that people don't like to see wandering around off leash.

Be careful as you socialize, don't let him meet every person and dog he sees. It can create a reactive dog. Socializing is more about a puppy being able to behave properly in public settings, and typically that just means learning to ignore everyone.

If you haven't yet, have your vet run a fecal. There may be something going on that's inhibiting his growth. Those growth charts aren't absolute, but I'd expect a male to weigh a few pounds more.