Baby won't sleep on back

Last week, at 16 weeks, my baby started rolling over and now it's done on purpose at night, I suppose for comfort. But baby HATES tummy sleeping and does nothing but cry until I come to the rescue. This happens at least a dozen times and I'm getting no sleep. I'll put baby on back, baby rolls over onto tummy immediately, I walk away, baby cries. Rinse and repeat all effing night.

Please what can I do to keep baby on back? Or at least side? I've tried rolling up blankets and propping, but that didn't work. I've tried extra milk at night to keep full, even nursing to sleep, and nothing is working. The only thing is baby won't roll when being held, but I'm not going to cosleep just because I'm not comfortable with that. Please tell me this is just a phase!! 😵