GSDs mature at three years of age. Did the vet recommend castration because of guarding instincts or because of over-possessiveness? There's a difference. All GSDs guard their family (herd) with their life. Also, you don't have to make him have the operation yet, without doing a test first to see if it will work. If the guarding/over-possesiveness is inbuilt, the vet can give them a 'dummy' injection first. If it works, then it's worth having the operation (because it is usually to combat over-possessiveness). If it doesn't work, then the proper operation won't work either and it means you will need professional help (with his head) from a behaviourist. The outcome of that isn't always guaranteed though; some are so screwed up, unforatunately, that they just can't handle life and all the responsibilities they (believe) they have to shoulder.
If you've never been loved by a GSD, you just ain't been loved; if you have, you know what love from a dog is (don't be naughty).. Trouble is, if you've been loved by a GSD who thinks it's his mission to guard and protect you because you aren't capable of protecting him, you are on a very, very sad, long road to nowhere if you can't sort it out. Please stand up for your dog and let him see that you can look after him; not the other way around.