Sudden selective aggressive GSD

Chief is 20 month old and we adopted him from a family 9 months ago because they had to move to unsuitable accommodation for a large dog.

They had him from 10 weeks old. He is KC registered, has a good pedigree, is hipped scored etc. He is very obedient (in the absence of low flying pheasants which completely irradicate his ability to re-call) he sits, waits, lies down, fetches, recalls, walks close, and will do most agility tasks on command. He is also 95% reliable in these commands from a distance. I have no reason to doubt his previous owner when I am told he has no previous bad experience of any sort.
He has always been friendly since we had him, with all people we meet and all visitors to our home. He has never shown fear and is a confident, happy dog. He has no problem with his food being taken from him etc. He is cared for equally by myself, my husband and my 2 sons (14 and 12) and appears to have no preference as to who he is with - although typically he prefers us all to be together. He is well mannered, walks behind us through doors, has access to the back of the house only, is not allowed on furniture, eats after us.

He has always been unpredictable when meeting strange dogs. Some he likes, some he doesn't.

Both my sons have friends round regularly, we a re a busy household, and he has always been friendly. Until 2 months ago, my son had a friend back who likes dogs, who Chief not previously met. He was his usual self for a while, and then I heard him bark and growl, and removed the child immediately. When I questioned the situation, it was ambiguous as to whether the lad had accidently trodden on his paw whilst petting him. He then went home.

About 3 weeks later, the boy came round again and Chief growled and barked as soon as he saw him. I assumed he simply remembered a bad encounter.

There were no further bad reactions to any one else so we decided he simply didn't like the boy.

2 weeks ago another new lad came round with my son and out of the blue the dog growled at him. Next time he came round, I sat the lad down with me, let the dog in and he sat at my feet, appearing to calm. He wondered upto the boy after a while and sniffed him and came back and sat down on the floor next to me. Assuming him to be comfortable, I allowed the lad to offer to stroke him - Chief snarled quite nastily - but pulling away with sideways eye contact rather than lunging forward. I immediately removed him.

I have no idea why he dislikes these to children but is perfectly happy around everybody else.

Can you offer an opinion on this?