I think the key to preventing bed sores and tail bone pain from happening is to turn and reposition your son every two hours if at all possible while he is lying in bed. Use pillows between the legs, arms, ect to make his lying position more comfortable. Also, it's important to give them pressure relief while he is sitting in the wheel chair, too. Use pillows to free float his heels off the bed to prevent heel decubs which can happen from pressure and friction/shearing ( i.e. pulling him up in bed). Check the skin on his sacrum daily for any signs of reness or breakdown. Notify report to his doctor any positive signs. There could be a decubiti ulcer developing. This will cause pain as well.
An air mattress is a good choice. Make sure there is air in the mattress since this device can deflate from time to time.
What about PT, massage therapy, range of motion exercises, or tylenol, to help reduce the pain to his hips and arms?