You don’t say how old he is OP. It’s always worth checking that he has basic quick recall skills safely under his belt.
Depending on his age this could be
number bonds to 10 and twenty ( including subtraction)
addition and subtraction over the “ten”numbers ( 30/ 40etc)
ditto over “hundred” numbers
multiplication / division facts ie times tables both ways
Having to stop and work out basic calculations from scratch every time slows you down, leads to errors and is very frustrating.
How are his reading skills? As children go through primary school reading a problem and working out what the maths they need to do actually is can be a stumbling block. Sometimes it is worth sitting down and just focussing on
what the problem is about ie what you are being asked to find out
what is the information you know
what are the steps, ie what is the maths, that you need to do to get to the answer.
Get him to verbalise his thinking, it can help to clarify the process. Sometimes I wouldn’t even bother with the maths at first, just getting him to understand the problem and say what maths he would do is enough. (You can give the same problem a few days later and get the maths bit done then, this also reinforces the thinking bit.)