you don't.. I wouldn't suggest a pillow/other bc it can fall on face impede breathing, etc. Just place on belly, when he rolls over let him kick around and then put back on tummy again..
Once my daughter started rolling I didn't do much to stop her/keep her on her belly. If she got mad from rolling over, I would flip her back. Now she can roll from front to back/back to front and spends a good amount of time on her belly still.
I wouldn't worry too much about it.
Meet DH in 2008 - Engaged on 4.11.2011 - Married on 9.22.2012 - BFP on 5.10.2013 - DD born via emergency c-section on 1.23.2014
Re: Tummy time/rolling over