I don't think there's a need to force the issue for more then a few minutes at a time. The benefit of a little tummy time is that after awhile they start pulling their upper body up to look around. It strengthens those muscles that will get them ready for creeping, and crawling. Just do it off and on throughout the day. He'll enjoy it more once he does start trying to move around. I used to get down on the floor next to my kids, with my face at their level. They really liked that.