My son was always getting funny marks and bruises at this spot on his back. The only thing I could think of was something poking him on his car seat but I checked and there was nothing. Then I watched him closely in the playground and realised that when he came down the slide he would fly down and lay back at the end and then land on his bottom. His lower back was hitting the lip of the end of the slide. Mystery solved!
Your child seems a bit young for this but if it looks like a bruise or graze I would be looking closely at any toys he sits in or rides on etc. Good luck working it out. X