My feeling is once a child can roll they can sleep as they wish. I also found that once they can get on their tummy they very quickly (with in weeks) learn to roll the other way, as it is easier to do.
Give him lots of tummy time during the day so he can practice.
Don't worry about the nap, let him be in his crib, the nap will come back soon.
In terms of night time, if you go in every time he ends up on his tummy you will be going back and forth alot and you will be teaching him that every time he cries at night you come which is a dangerous president to set. On the other hand there is something wrong, he is not crying just to get you to come, so you are stuck (as is he:)).
When I had this problem I went and turned the baby over, but then I found after the baby learned how to do it his/her self I had to re-sleep train (cry it out again), but luckily the second time you do it it is much shorter of a cry it out.