stomach sleeping

What's your opinion in baby sleeping on their tummy. My son is a month old and I laid him down in his crib for a nap after he fell asleep on me. He woke up within minutes (which is normal for him), but I rolled him onto his tummy and he fell asleep again instantly. He would only sleep on his back for about an hour, but could sleep on me for around 3-4. I have his monitor set up and am watching/checking on him constantly.