I would very scared to use the Boppy in the crib!!! Even in he is not rolling, he could simply slide down and end up with his face under the edge of the pillow! I would be much more comfortable with the many suggestions of putting something UNDER the mattress to elevate it - that makes sure that there are still no soft surfaced or pockets that he could get his face stuck in. He's still too young to wake himself up and reposition if he was to get his face stuck somewhere.
I hope I don't sound too strong against it - but I'm a big safety person, and can't stand the thought of anything that could put our beautiful little ones in danger that they don't need to be in. As you know with your older ones already, they will find enough ways to put themselves in danger as they grow and explore:)
Good luck!
S.