Rachel, it seems like this is a hard habit to break, but it can be done. Keep in mind the reasons he might like this position. Might have a little belly ache, might like to sleep on his belly, might like the warmth. Some babies like the warmth they feel when held. Does he sleep well at night? I noticed one of my kids (have three) used to kind of get that cold feeling or something..I would place a blanket under his body while i held him, then when i put him down, the blanket was still in that same position next to his belly and it was warm, so it didnt feel so different and was warm. Of course, he wasn't old enough to get wrapped up in that blanket, it was one of those thin ones and it was a routine only for a while. It may all boil down to, are you willing to let him cry a bit when you lay him down? You have to be willing to let him cry maybe a bit. He NEEDS a nap, you know that, but you also know he can't be held during the entire nap. So, it may be a warmth, cool bed situation, it may be too light in his room, etc..or you may have to just bite the bullet and let him go until he falls asleep..It sounds cruel, but think about it, we don't fall immediately asleep when we lay down, and we have learned to think about other things until we nod off. What does he think about a mobile, my kids used to get interested in that and nod off. Or play with a toy that was safe while they nodded off..Goodluck..