But it's not just that he doesn't take treats either in training, he just seems very disinterested in food in general. He might every once in a while take a treat from me but if anyone else tries forget it, he wouldn't even take raw meat from people who didn't believe me and tried. Could that be related to stress?
Yeah, that sounds exactly like my dog. She has turned down hot dogs, raw meat, even steak from strangers. It could definitely be related to stress. As long as he is not underweight, I would try not to fret over it too much because as you say, he will pick up on it and not want to eat.
Another tip someone gave me was making a "trail mix" out of kibble and training treats of various sorts and feeding that throughout the day as a reward for good behavior. You can set aside the kibble you plan to feed in the morning and mix with training treats as needed, then whatever is left feed at the end of the day. The variety builds interest in the food.