I don't know, I haven't figured that out either. DS just turned 15 months and is still very much a baby in that he wants to lay all over me, be held, be carried, etc. And he recently learned to climb so he's always getting on the couch and chair and using my belly/boobs to pull up or land on.
I've just been telling him "no that hurts mommy" and sitting him on his butt next to me. Doesn't really work especially because he has no clue what a baby brother or sister is! I hope you figure something out, sounds like you're in a really tough spot because of the previa!