Three Year Old Picking His Gums Until They Bleed

I have a three year old who is developmentally delayed, but has not been conclusively diagnosed with any specific problems. Over the last several months, my husband and I have noticed that he was picking at the gums of his lower eye teeth while in his bed. Usually he is half awake/half asleep when he does it. The gums there are getting worn way very badly and are probably 1/8 inch lower (at least) than ever where else. Sometimes we find blood on his face or sheets. We are putting socks on his hands to stop him...but it doesn't seem to be solving the problem. He likes the socks, but still can do damage even with them on, especially because he is sleepy and doesn't really know he is doing it. What should we do???
Thank you