One thing I've found after looking into dcalc and dlex is they are very individual problems, unique to each sufferer. It's a little like the common cold, just as two people with a cold can actually have very different virusses causing the symptoms, two people with dyslexia could be manifesting similar symptoms for entirely different reasons.
In my case with dcalc, I have a short-term working memory, meaning I usually can't hold all the stuff I need to in my head. So the moment I have to carry a digit, or perform more than one step, the numbers just drop from my head. You know that feeling of overwhelmingness that you get when mentally doing a sum or calc that's just too big for you? Well I get that much, much earlier than other people.
However, because it's basically a memory problem not a maths problem, my comprehension of maths is unaffected - so long as I have a calculator or a sheet of paper to make up for my faulty memory, I'm fine.
In the case of dyslexia, a student can understand number concepts, but I should imagine they'd be more likely to prefer practical maths. So dyslexia needn't necessarily prevent a man becomming a carpenter and doing all the necessary calculations, yet not being able to read a simple paragraph (I know a guy in this position).
Dyslexics that I know tend to be more practical because the brain makes up for its weaknesses. So you could have your character 'seeing' objects in his head while he calculates, splitting them to divide, an so on.
But understanding complex mathematics could be difficult, you may be better off making him just stupidly good at mental arithmetic. Maybe as a child, he's given books to look at, but he innately prefers clacking away on his dad's calculator, and piling lego bricks into orders of size and stuff.