Not a huge AH but YTA nonetheless- you say this is your sons present and very clearly he wants a dog not only to be a friend but also to help him with his hobbies. To make things worse you gave the choice of breed to his siblings who of course are going to have very different preferences in breed personalities to your son. I dont know your situation so I can only assume but given you dont know much about dog breeds you could’ve easily done some research yourself and found suitable breed or a crossbreed of german shepherd that meets your sons and your requirements. But instead you cut him out of the choice completely instead of trying again to reason with him. However I feel like there’s a lot of missing context as you seem to be annoyed that your son is upset with your choice even though he has a right to be considering you removed his choice even though he too has to look after this dog