I think that he will do it in his own time, but here are some games I played with my son.
-I would crawl around on the floor all around him. I would pretend to be different animals and make animal sounds. He would laugh so hard and we would have a wonderful time.
-Buy him some overalls. Pull the back of the overalls when he is on his belly and help him learn how to keep his belly off of the floor. Pulling the overalls will keep his belly off of the floor.
-Put him on a boppy on his belly. We used to watch a Baby Einstein dvd while we did this. I am not a big advocate of children watching tv, but this will keep his attention at least for a few minutes and help build his muscles in his neck, shoulders and arms.
-I used to lay on the floor facing my son when he was having tummy time. I would engage him in different activities like singing songs, playing with his toys, etc. This encouraged him to play on his belly.
-Play chase with your husband on your hands and knees. We played this all the time and my son loved it. We used to play it will my nephew who was 20 months older than my son and I think this helped the most in teaching my son how to crawl.
I did all of this when my son was very young, like two months old. He first started rolling everywhere, then he did the army crawl and eventually he crawled on his hands and knees. He only crawled this way for about a month before learning to walk. Keep encouraging him and have fun with him. Also accept that he might not crawl the way you want him to. There are so many different ways to crawl. Crawling IS a very important development as research has shown that it improves speech and reading skills. Have fun and rejoice in all of his milestones!