I agree. Have him feed him, hand feed him if possible. It sounds like he is already trying to spend as much time as possible with Foster, which is an excellent start and the right move. I might suggest also having DH carry treats around with him and randomly give them out throughout the day. Try having him start basic training with him on weekends, even if just doing a refresher course. It will help to strengthen the bond b/t himi and hopefully he will learn that not all men are evil.
If possible, I would also try having other men offer him treats, such as a brother or friend to help reinforce the "men are good" thing. Or when and if he goes up to a man friendly like, give him a treat. I hope he is food oriented, or it makes it a bit more difficult, although still managable.