My son fits this bill exactly. He is nearly 16 and has never had any help at secondary school ('good' London comp).
After his primary school denying that anything was the matter for years, and him getting increasingly unhappy we took him to the Dyslexia Institute at Staines at the end of Year 5 - to find out that he was not stupid as he thought but in the top 2% of the population in IQ terms.
He got level 5s for science and English, high level 4 for maths. Secondary school said no help needed as he was coping, although they did put him the G&T group.
He did pretty well at school by working hard until he got to about Year 9. Not very well socially as he tended to have one friend at long intervals and they all left, one after the other. In Year 9 he made friends with a group of bright boys but I think by turning himself into the clown - and his work suffered, but I was pleased to see him getting more socially able.
Year 10 and 11 he started out with predictions of all As in 11 GCSEs. I hadn't wanted him to do so many - his sister did and it was a nightmare of stress. BUT the school were completely unhelpful. He wanted to do triple science - loves science and is really good at it. The damn school said that the only way he could do all 3 was to be in the stream that did 11, and that he had to do a language - if he didn't do the language then he wasn't bright enough to do 3 sciences.
Here we are just before GCSEs in Year 11 and he has voted with his feet - failed to complete coursework in 3 subjects and is supposedly focussing on the 8 that he needs to go to 6th form college. But only today we had a massive row about revision ... and his likely results are down from A*s and As to Bs and Cs.
Looking back, what would I do? Fought harder for him not to do so many - I still underestimate the amount of stress being dyslexic causes him. Been much more aware of the perils of coursework, and more in touch with the individual teachers about what he had to do and when - basically he lied to us and got about 6 months behind in certain subjects. One teacher called him lazy - but for me it was all bound up with the dyslexia and the kind of self hate we had way back in Year 5.
Can a gifted dyslexic child succeed in an ordinary secondary school? If they have huge reserves of determination, and if you help them better than I was able to. Try to get the school to focus on their actual needs - 8 good GCSEs - rather than just fitting them on a one-size-fits-all track. If they have an idea of what they want to work towards it may help.
If we survive GCSEs I'll let you know ... He still has very low self esteem, and thinks himself a failure. The trouble with knoiwing that you are very able is that it sets the bar very high - but the dyslexia is still always pulling you back. I still try to push him to do too much. Huge rows about it all. Wish I were a better mum .....