side sleeping

Stephanie • My names Stephanie, I`m 25, a wife and mom to two kids, and expecting #3!
My son is five and a half months old and recently began sleeping on his side. Even if I place him on his back he'll immediately go right back to his side. They say not to let them do that but if he's got decent head control and cries when he's uncomfortable(especially on his tummy) should it be okay!