Keeping them on their back

I've been grappling with a 235 pounder (I'm 135) who I'm much more technical than, but I've been having problems keeping him on his back while I'm in his guard, lately. What generally happens is that he'll be on his back and he'll just sit up and go to his knees, and at that point I just usually fall back to guard because when we go head to head, he kills me. So what's the best way to keep him on his back when he tries to sit up?

keep his feet off the ground.

yep, andre is right on the money.

When he sits up grip the inside of both of his knees and drive them towards his chest, that will put him right on his back

Hah, thanks. That's pretty obvious. I should've thought of that.