As much of play is a form of play fighting, it is not unknown for owners
to mistake play behavior for agression. If you have any doubts about
your judgement it might be useful to have a more knowlegeble dog person
accompany you with your dog to verify that he is truly aggressive.
Still it is likely that he is aggrssive, and too big for you to handle
unless you become very good at training him
Puppies are almost invariably good with other dogs, but that often
changes on adolescence (7-8 months for most breeds), or adulthood 14-16
months for most breeds). There are some breeds where agression is rare,
but German Shepherds are not one of those breeds. Aggression tends to be
less common with males than with females, but it is far from unknown
with males
At this stage it is likely that the best thing for both of you would be
to turn him over to someone else. Keeping him in the yard is not a good
solution. If you are to keep him, both of you need training. He needs
to learn that you are the alpha male, and he needs to pay far more
attention to you than to other nearby dogs. You need to learn both how
to maintain that attitude, and how to read his behavior so you can
anticipate and prevent potential problems.
Best would be one on one training, by a knowledgeable dog person. You
have ruled out a professional, but I urge you to reconsider. A few
sessions with a professional would be a minor expense compared to the
years of food and health care you can anticipate for your dog. If you
still cannot afford a professional look for a local dog obedience club,
and ask them for advice on dealing with him. Five months ago I would
have suggested obediance classes preferably at a local obedience club.
But given his current strength and dog aggressiveness, starting off
training him around other dogs is problematic. There are training books
and videos that can also help, but they are not as good as one on one
training, and it will be difficult for many people to be confident that
the behavior they get in the house and yard will carry over to the
street in the presence of other dogs.