Head banging

My son is nearly 15 months old and I’m getting concerned with him sleeping in his cot. He constantly turned onto his front no matter how many times I flip him back over. He’s only been doing this recently. Also he rocks on all 4 whilst he is still asleep and he is banging his head on the cot all the time. I do have a bumper around the cot. I’m constantly waking him up to stop him rocking and flipping him back onto his back. I’m hardly sleeping because I just watch him sleep to make sure he’s fine. I have noticed when he’s sitting up on the carpet he’s starting to rock back and forth aswell. Is anyone else baby doing this or should I ring his midwife?