You can be there for son and let him know in words and actions how much YOU love him. Not much more.
15 is a very difficult age. My daughter got into trouble at school at that age as well. She struggled so much with what happened to her when her dad left. My kids have me. The like my new BF. They spend all holidays with us. Their dad has now asked them to accept OW and spend some holidays at his house. They have just laughed him off. D says "As if I could just forget that OW broke up our family and I would want her in my life. Not happening!". I tell the kids that I will support their decision on it no matter what.
I understand everything that you are saying. Wish I could tell you it would get better but it's not likely. And not for a long, long time.
