Same problem here with ds. Massively underperformed while at school (been home educated now for six years), largely because he wouldn't commit to task. For him, a large part was executive function, not being able to organise his thoughts and put them to paper. It also depended on the task - he really struggled with English work because it's so creative, no right and wrong answers as in Maths or Sciences, and didn't dare risk getting it 'wrong' as he perceived it. But with those factual subjects he'd breeze through. As someone above pointed out, personal interest in a subject makes a big difference to how likely a child is to engage, so worth bearing in mind.
We've had an amazing tutor whilst home educating and she sat alongside him as he worked, giving guidance and prompts when he got stuck and couldn't move to the next steps. It helps that he types all his work now (he is also hypermobile and handwriting is painful) because he can write down thoughts as they occur to him and doesn't have to work in a linear fashion - he can edit and tidy up till he is satisfied with it. In English or History exam questions, he often found it easier to start with the conclusion and work his way backwards, which he wouldn't have been able to do if he'd handwritten. She gradually withdrew her support until he could work alone.
I get what's being suggested about waiting till he's 'comfortable academically', because ds's confidence certainly grew once he'd got a couple of exams under his belt. He copes well under exam pressure and once he saw what he was capable of, he began to feel more at ease with taking risks. However, I think it might work better as a two-pronged attack - working on supporting him now with his current abilities, and helping him to focus and engage, plus being prepared to change strategies as he develops his confidence with maturity and experience.
Which reminds me of another aspect. Ds is very self-conscious of his work and isn't comfortable having other people see it. This includes me and to an extent his tutors. In an exam, he could write freely knowing that neither of us would see his work and the only person who would was an anonymous examiner who he'd never meet. So consider that your son might not want to share his work and that could be holding him back too.