I am a foster parent and I had a child that did this, she was 1 year. I worried because the Children's Aid Society (the Canadian version of Social Services) saw her on a regular basis. I reported it to the agency and they had no issue with it. This lo didn't bang had enough to get the bruises that your lo got. She did this when she was frustrated during the day. The banging started on the carpet floor, then moved to the tile floor if it was available. We tried keeping her away from the tile floor so she would use the wall. She eventually ended up leaving so I am not sure what happened and if it stopped.
I would just say that if he is doing it in the crib, try to get something to cover the area he is banging so that there is padding.
I hate to say it but it's a good thing he is doing it at daycare. Since they see it, they understand what you are going through and are less likely to report it.
I also hate to say it, but if you are really worried about social services, can you get the banging on video? It's sad to say but you have to protect your family. You want to be able to show people what your are going through should anyone question you.