My son is 4 months old. He was sleeping in his crib from 2 weeks to 2 months old. I moved him back…

My son is 4 months old. He was sleeping in his crib from 2 weeks to 2 months old. I moved him back into our room because our monitor kept breaking so I wouldn't be able to hear him at night if he needed me. We just got a new crib and it is in our room. He will not just go to sleep for a nap or bedtime anymore. He just cries. He won't sleep if we rock him or anything, he just cries himself to sleep. He does stay asleep for 12 hours once he's asleep at night but him getting to sleep is hard on all of us. I want to be able to help him sleep but the only thing that helps is putting him on his tummy and I just can't let him sleep unsupervised on his tummy. Any advice??