This is heart breaking. Do get help (if you haven't) for your son (and for you to help him, and you to talk about these feelings & ways to interrupt them)----------what is frightening is that the time between someone considering self harm, and engaging in the activity is brief-----
I would have an ongoing conversation with him about how he is feeling, and get him involved in some activities with others that he can feel GOOD about----
It is really difficult for a kid (or adult at times) to know that the "life sucks" feeling is not forever (even if it seems that way at the moment)---keeping a list of Good things/thoughts/can be helpful. And, of course, knowing that he can talk to you---keep him making pictures-----------he seems able to express his feelings this way.
Take good care. Many hugs((((((((((((())))))))))))))))))))