very odd that he was stable then changed. at his age now its obviously the way he's going to be. some do go through fear stages at that age, but usually those ones also show a fear stage earlier too.
does he tuck tail and try to get away from strangers? some will do that, some will show other warnings like growling and backing up, or run away. definitely conditioning him slowly will help, but it does take time. meeting people i would have them ignore him, no touch, no eye contact, etc. he may after a few minutes after he feels no threat go smell the stranger, and thats good, but still i would not let them touch him. i would go through a time with that process and see how he is, then maybe add food for the person to give him, still with no touching, eye contact etc. long process, but does work. to what degree his fear is, time will tell, you might always have to deal with somedegree of it. in the meantime i would continue with a good trainer and class as long as you feel he is progressing in that setting and its not to overwelming for him. to much to soon can set them back.