S.S.
Easy solution...put it under the fitted sheet and mattress pad (will save it from some of that spit-up too!!!). We do that in our son's toddler bed so it doesn't move on him and he can't get trapped under it, but still lets him sleep on it. If the regualr size pillow doesn't fit well, we bought a travel size pillow at Target which fits great or you may have to sew together 2 sheets together if he will only sleep with the big pillow!!! As far as safety, I figure if he's been sleeping on it fine so far and it's helping him sleep, I'm all for it! Just check on him a few times at night and use your gut. Good luck!
S., 26, mom of Tyrrone, 14 months, and #2 on the way!