Don’t get too hung up on doing “ written sums”. As someone else said go back to basics. Make sure he is really confident with simple basic counting and understanding numbers, start with numbers under ten. Try to get him to understand that he doesn’t always need to start counting from 1. If he sees four apples, can he tell you there are four without going 1234. Practise doing this until he can accurately tell you how many there are using objects up to about 6. Put the objects in different arrays ( an array is a way of arranging objects, think of a dice) use different arrays. If you tell him there are four objects and then you hide some, can he work out how many you have hidden. All this is helping him to understand that we can group numbers in different ways but the number stays the same. So subtly teaching him some basic number bonds , so he understands that five can be five objects in a row or in a circle, it can be four objects and another one, three and two, two and three, one and four and so on.
Then start using larger numbers of objects, but only up to ten. these will be harder for him to recognise at a glance, so teach him that we can group larger numbers to make it easier to know how many we have. We can make ten into two groups of five, six and four, four and six, three and seven , try to encourage him to only group into two groups , though if he says “I can make ten with a five a two and a three” then praise him because he is beginning to really understand.
You could then start to encourage him to label the groups with numerals, not writing a formal sum, but getting him realising that he can represent the physical group by an abstract concept , a written number. When he is confident with this then slowly start to use formal written forms for simple sums, but make sure he can use physical objects if he needs to.
I would also use a number line, and show him how numbers look and how we can use a number line to count on in ones, twos, fives etc.
We sometimes forget that it took the human race thousands and thousands of years to develop a number system that works as efficiently as our system does - unless you are a computer, when you need a different number system altogether - with a few very odd ones along the way, (try multiplying with Roman numerals, impossible until the Greeks thought up the concept of numeral 0 )yet we expect children to be able to do this in a couple of years , then wonder why so many find it hard.