he is probably saying he is going to be a famous actor to avoid thinking about failure - I'm sure he doesn't think he is going to be a famous actor - its a way of heading you off, especially with the school work an issue.
He sounds a bit depressed, is there anything you can do to bolster his self esteem that doesn't involve reminding him that he is disorganised? Just some little activity he can do where he feels good about himself, rather than any demand?
Ds has dyspraxia and at 15 did a lot of that faux I'm going to be a famous footballer, I'm going to be a celebrity, I'm going to be a star. It was ridiculous he was terrible at sport and very shy at acting. It turned out that he wanted to think of a time when all his classmates would admire and worship him, instead of regarding him as rather stupid and boring.
So I suppose we had to remind him that his boring self was worth befriending spending time with, not this celebrity self.
Ds liked films, and going to films. He also likes singing and is very good at it (but refuses to join extra curricular choirs/orchestras outside school) these things increase his morale, we don't press it on him that he is good at this things and has to succeed with them, more that we don't "diss" them in favour of other things he should be doing, like be tidy or sociable or energetic.
Ds is very bad at school work, he himself made the point that decidedly average is still a pass. There were children in his school who did not pass their GSCEs, I think it reminded us that ds was still lucky to be able to go on a 3rd level course at sixth form, AND that if he hadn't passed most of his GSCEs he would still have been welcomed by many many other vocational courses which only wanted English and Maths. If he had done carpentry or catering it would not have been the end of the world, in fact it would have been great if he had wanted to those things.
Getting a job is a long way off, and I think it makes them feel very tense to be reminded of their long term unemployability. Plenty of people get jobs in the right fields, when they grow up, fields their parents might not have considered, but which suit them - there is a whole world out of there, and academic qualifications are only a small part, often children get put off because they are no good at school, but in a different setting they find their metier.
I suspect your son has some form of inattentive ADHD and Dyspraxia not just dyslexia, and his confidence is severely affected by this. He won't want help because his self esteem will be bound up in trying not to think about his failings.