N.E.
Ok this is an outsider looking in but these are my perspectives
He is your "son" regardless of whether you gave birth to him. When you married his father-you chose the whole package-not just a portion of it. He is in your home during the school week so it is your responsibility to him as a parent to lay the ground work for getting a good education. You have to be the proactive parent. If not you could possibly be paying child support until he is out of college.
$700 a month in child support is nothing. I take it that his mom is buying his school clothing and furnishes things like insurance, medical care, etc. I see some frustration that he has all these "gadgets". Is it that you cannot afford those things for your 15 year old and it is have some negativity with that son?
Be careful when you go reporting her to a state agency-it could backfire on you since he is in your care through the week.
I am not trying to be rude or hateful, however, you are coming across as a bitter step parent and that may be the same impression you are giving Beau