“Back is best”

I know that when you put the baby down to sleep he should be on his back. But my son WILL NOT sleep on his back on a flat surface at least, maybe with some covers and a pillow but he will not, he fights to get to his side and will sleep just fine there, or he’ll lay on his tummy on our stomachs and sleep just fine but swaddling him doesn’t work and just laying him on his back sends him into turmoil. He hates being swaddled so nothing really calms him aside from a titty. Am I doing something wrong? Is it safe for him to be on his side? He literally won’t sleep any other way? Help, any tips?