N.W.
Once my daughter could roll to both sides (7.5 months), she was more comfortable in a flat crib. She still has awful reflux. She naturally rolls onto her right side--a position which helps keep her stomach acids in her stomach because of the shape and position of the stomach.
She's much happier sleeping on her side or even her back on a flat surface than she was sleeping at an angle once she got more mobile.
she's on zantac and reglan and prevacid.
If your son is fine on the floor during the day, he'll be fine on a flat crib.