If he doesn't believe you (and that's not unusual) then look for opportunities where other people will tell him he's good at things. Sports teams, clubs, organisations like cub scouts and other group activities are terrific for boosting confidence. Partly because they get told 'well done' by people they look up to, and partly because they can have a go at new experiences.
Do you give him detailed reasons eg you did that well because of x, y and z. That seems to please my children more than something more general. And it can be a good way then of encouraging improvement without being critical, by highlighting the good bits that you would like them to do more.
DD2 was like this. Would cry if we said she was good, or clever, or pretty because in her mind she obviously wasn't and why would we lie to her. Eventually we ended up at educational psychologist. They suggested a few things. Particularly useful was the 'I like x because' game where we would go round the table and each of us had to say why we liked another person there. DD had to say about us as much as us about her. It did have an impact. We did it every day, she didn't think it was targeted at her and gradually the message got through.
She's now 18 and is very aware that she is good and pretty and clever...
I have read some interesting research about this... mostly about how it's much better to praise children's effort than tell them they are clever/good etc. It (apparently) helps them to persevere, whereas children who are constantly told they are just good at something start to feel the pressure of having to keep it up. Also as others have said, be specific, not general. So: 'Wow, well done, you wrote that so neatly! I can see you were being really careful,' or 'I'm so proud of you - you kept trying to work out that maths till you got it right, and you didn't give up, and you GOT it.' I have been trying hard to do this (so praise me ) and have found it takes a bit of thought compared to the things I used to say. Getting easier though.
Meant to say that ds1 was another one who would reject praise outright, seemed to have very high standards for himself, and little by little he seems to have relaxed into the new way we have been talking about his achievements, is more prepared to accept that he's done something well.