I can so relate to you. My ex-husband abandoned my son when we divorced and he so needed him in his life. If you son is the younger boy, maybe big brother can help. If he is the youngest, maybe granddad can help.
I think he is crying out for attention from his dad. My son did similar things as this. Be sure to keep the communication open between the teacher, your son and you. Don't fail on this as this is a bit of a test for you and the teacher by your son. Every time you set something up to follow up on him, he is testing you again. This probably won't be the end of it as he will continue to come at you with something else. When there is a divorce, children always blame themselves (If I hadn't done this or that, Dad would still be here). His Dad already left him, so now he is setting himself up for when you leave. You might consider some counseling for him and maybe for both sons and you, even talking to your ex to see if he will give the boys some time and attention as they still need a dad in their lives, he divorced you - not his sons. Until this void is addressed by the ex, granddad or whomever, you will probably continue to have problems. Kids have a hard enough time as it is without Dad running out on them as well. As you know, children need stability in all parts of their life.
My biggest suggestion is even when you are frustrated as all get out at him, take him in your arms, give him a big hug and tell him how much you love him no matter what and that you are going to be there for him no matter what. Also, just because this is the son giving you problems right now, don't forget about your other son - he may not be giving you problems that you are aware of, but he still needs you too.
Good luck and God bless!
Deb