At age 14 many diverse issues are going on with most kids. Their hormones are raging, their personal idenity is evolving, their bodies are changing, etc.
Thus he has much going on. However, where school work, home work, projects are concerned, I see you mentioned many things that you have tried to do, except for 1, sitting down and actually helping him complete his school work. The subject may not interest him, bores him, or he may not excel at it, thus avoids it. It may also be an unconscious effort to get attention from you, negative or positive it's attention.
Therefore, as a former teacher, know that he may just need some consistent 1 on 1 attention and help with his school work, and it is up to you to not just offer it, but to actually sit him down, help him, keep him focused until homework or a project is completed. Even if this means sitting down with him, in a quiet spot each and every night, rather than a spot where the Tv is on, where a baby or other child may interfere. Praise him when he does Ok-well, and do so often as praise helps build self-confidence, punishment just confirms lack of confidence.
Also when he feels worthless, stupid, unable, and you tell him otherwise, he won't hear it, it must be shown and proven otherwise. So help him, as talking about it won't get anywhere.
Getting him a tutor may help, but A. he may not accept one, and B. you as his parent have to reinforce the lessons at home, praise mom, not talking and punishment!
No one ever said being a parent is easy, for it is not. Our kids need us and our positive attention. Just because they are bigger, and seem so grown up, they are still kids and need us whether they know and accept this or not.