Ds2, the nt one - who was 8 last month, does this. He also reverses several letters (b/d, p/q, s, e, j and l) and occasionally mirror writes as well - particuarly his own name for some reason. 
I've raised it a few times with the school and they just do the usual of telling me he is young and will probably grow out of it. 
Like your ds, he has no problems working out the actual sums though - he still knows what number he's written and gets the answers right.
We have recently realised he's worse on computers, two digit numbers are typed backwards, so 21 would be entered as 12 and he can't distinguish a decimal style 2 from a 5 on say, a digital clock or scales.
He has been getting slowly better with it this year though and can now at least see he's reversed them and go back and make corrections, whereas last year he had no idea he was doing it. It has taken a lot of support though, with myself and his teachers going through work with him and showing him the reversals until he started to be able to see them himself. Now we can just remind him to do a final check before he hands it in and he gets about 99% of them.
He still needs a guide of the correct letter and number formations in front of him when he's writing though. He will often still copy them wrong, but is more able to recognise the reversals afterwards if he has a correct example to check against. At school he has developed his own strategy of finding the right versions on the notice boards or classroom displays, or he uses his number line or number square. At home we make sure he has a ruler for his number formation and keep a book open on the table for him to use for letter formation.
In ds2's case, he lost all his letter and number skills after a serious illness when he was in reception year and had to start from scratch in year 1. I wonder whether this means that, due to effectively starting late, he is just delayed in developing concrete skills in these areas and his teachers might be right that he will grow out of it. I have heard, although I don't know how true/accurate it is, that up until the age of 8 it's still considered normal for children to reverse letters and numbers. I wasn't happy and didn't agree with this at all until recently though, as I felt he wasn't improving, but it does seem that he is really coming on with this now and I'm hopeful that they might be right and he may grow out of it.
That said, if he is still reversing by the time he goes into year 4, I will be pushing for him to be screened for, well I'm not sure really, dyslexia/dyscalclula I suppose?