Talk to Pediatrician on 5 Month Old Can't Maintain Sidelying Position

My baby is going to be of 5 months on 26th. He hasn't rolled over in any direction yet. Due to flat area on his back of head, i was advised to put him in sidelying position. But he doesn't maintain it, neither while sleeping not awake. His upper arm just moves randomly and he can't bring it in front or midline and he starts fussing and just rolls to his back. Is this normal behavior? He hasn't even rolled to his sides yet. Neck holding is proper. He can sit in tripod position by himself also. He is a bit chubby so can that be the reason of not rolling over yet? Please tag any developmental pediatrician.