Our Griffonshire is the exact opposite and loves everyone, he will leave with the mailman or anyone that comes to the door if you let him. How old is he? Has this been a behavior that he exhibited since he was a puppy as well? Ours is 6 months and we try to get him out regularly to be around other people and dogs so that he does not become nippy or scared of people.
I think it may be normal behavior. I have 2 Griffonshires they are from the same litter. The runt is shy and hides behind me around anyone. The other soaks up the attention and loves anyone she comes into contact with. They both have been raised the exact same since I got them at only 5 weeks old (mother died sadly).
This may be a trait inherited from the Brussels Griffon. Many Griffon's included my own, fully believe in stranger danger. Mine hides behind me and pokes her head out between my feet and barks at strangers. While she is not aggressive in any manner, she does not like to be touched by people she does not know and will run from them.