DD is 3.5 and has Down Syndrome so I have been doing facial exercise, oral-motor-exercises with her for a while now.
Does you DS drink from an open cup, if not encourage this and the use of fun straws. I can recommend the water fountains, made by vogue I believe, and sold in Sainsburys,Toys'r'us..
Use musical instruments, party blowers, bubbles, all to encourage blowing.
I even have a little daily ritual of washing dd's face and massaging her face, chin, around her ears while applying cream, at the same time we pull faces, eeeee,oooooo, aaahhhh. You could even do this while brushing his teeth.
Use a mirror, so that he can see himself and you to play fun , copying games. I have even spread chocolate spread and 100s/1000s around her mouth and tried to get her to lick them off.
Encouraging the use of the muscles in fun ways has certainly helped dd, I hope some of this helps!