My advice is the same instinct that I've had for the last sixteen years: if a child tells you that there is a problem, then don't ignore it. Even if he only wants attention (which is highly unlikely for a 16 year old!), then its still a need and you need to respond.
To me it sounds like your DS is slightly depressed. Maybe not clinically depressed but feeling the world shifting under his feet and not able to control what is happening to him (losing certain friends from his day to day life , losing his free time, maybe losing his status as the clever one in a mixed ability cohort? etc). i think the way to help him is what CBT would do: just help him teach himself to identify what is making him uncomfortable and then rationalise it inside his head.
I think most of get the "but other mums don't make their Dc do x,y,z". I used to respond to that by giving my DC the choice (with associated consequences) but nowadays I just ask them to identify their own options and decide for themselves which path in life they want to take, adding that I will love and support them always and whatever they choose. Usually when they feel like they have a choice again, they make a sensible decision.
DS1 has started to try to blame me for the things he ought to do but doesn't want to. He says I will nag him if he doesn't, so he might as well just get on with it. I never let him get away with saying that (because I think if he comes to believe it, it will sour our relationship). So, i always rebut it, no matter how boring it is to keep repeating myself.