Sorry, I only just saw this.
Lisa it sounds highly likely that he's cutting. If they were random scratches he wouldn't have tried to cover up. First BIG clue.
Definitely tell. Also, chances are if you confront him about it he will get very agitated, very upset and deny. It might take a professional to step in an say something, to get some sort of response.
When easy child 2/difficult child 2 was cutting, she always wore long sleeves (or something covering her arms and would not talk about it. We couldn't remove all knives because she'd either find them or buy them (she was using a Gothic-style knife she'd bought as an ornament, and then sharpened).
She stopped cutting at about the same time as she finished school. Also coincided with her first boyfriend - not sure which is why she stopped. I don't think she knows either, although boyfriend did know and do his best to stop her from cutting - she simply felt no need, anyway. I do think school was a major stressor for her, major trigger.
She now has permanent scars.
Cutting - check the direction of the cuts, if you can. Dangerous ones go along the arm, effective-looking but not so dangerous go across. A longitudinal cut is one which can rip open an artery and do serious suicide attempt stuff.
Not all cutting is suicide attempt or even close - easy child 2/difficult child 2 was cutting to try to balance emotional pain with physical pain; if she saw blood she knew there must be physical pain and it felt balanced.
But you've seen it, he knows you've seen it so will be expecting you to tell someone. Part of him probably wants to talk about it, at least with someone outside the family as a counsellor. easy child 2/difficult child 2 did, but didn't know how to aks for help. difficult child 1 was more willing to talk about it (made less effort to hide it - he kept forgetting). In difficult child 1's case his emotional pain was so extreme, and he felt so bad about himself, that at least partly his cutting was a form of self-punishment.
Good luck with this one.
Marg