Viva variety! Offer him different ways to lie or sit when he's awake. Rather than just keeping him in one position for most of the day, include tummy time, back time, and stints in a bouncy seat, car seat, and front carrier. This will allow him to exercise muscles all over his body.
Reach out. When he's on his tummy, surround him with enticing toys. He'll get lots of practice reaching for them, which will help with the coordination needed for sitting up.
Play games. Dangle something alluring above his head so that he works his neck and torso muscles by looking upward.
Hit the gym. When he's on his back, offer him a baby gym so he can work on swatting, grabbing, and developing upper-arm muscles.
Learning how to get his body to do what he wants can be a tedious process for your little guy. But continued practice and your loving responses will help him become a sit-up sensation.