okay first, he's 5 months old. He's not being aggressive. he's being a pain in the butt. You need to step up the training on a big scale. Get with a trainer who KNOWS GSDs and understands them.
Second, yes, you did mess up by taking his food away when he would growl, therefore encouraging him that YOU the leader, cannot be trusted.
Third, get him THOROUGHLY checked out by a vet to make sure there is nothing wrong. No pain feelings that could be triggering problems. Make sure that puppy is 100% healthy. Make the crate a positive place. Leave the door open for him to come and go as he pleases when you're home. Learn a share it game, also known as a can i trade you game. Basically he has something but doesnt want you to have it. Trade him something of equal value and praise him when he reacts approproately ie. no lunging, growling.
ENCOURAGE TRAINING!!! Show him that working with you is a positive and BUILD YOUR BOND WITH HIM! And stop messing with him when he has food. Thats just messed up.
The ONLY time he should get treats or chew toys or meals, should be in the crate. The crate should be his safe zone. In his crate he is allowed to chew these awesome items or eat his meals without being disturbed. If you want to get the awesome items, remove him from the room nicely and remove the items when he cant see you. Make all interactions positive and fun.
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