Hey! Have you tried swaddling him so that he feels like he's being held..
My oldest slept on his side for about a month, I was OK with it because he didn't really move very much at that point and he co-slept with us so I felt comfortable letting him sleep that way. It was either that or sitting up in a reclined position like in a swing or bouncy chair, which at night didn't work. But there are reclining pillows you can get that help keep them tilted up slightly and they just go under the crib sheet. I think for my son, he had such terrible digestive issues it hurt his little stomach to lie directly on his back because he spit up ALL the time.
My daughter never slept on her back until now that shes 1 but i kept her bed clear from chocking hazards and shes been good it is dangerous but as a mom duty you gonna have to watch her close it took me a year and sometimes i gave in and slept were human take turns with dad luckily nothing happen good luck