I'd start with the very basic 'Numbers' app by dragon box. It seems very simple, but it gives a good visual aspect to the numbers up to 10, how they combine to make each other, etc. And getting that really embedded in, even for an older child, is very helpful.
You can then move on to 'Big Numbers', but I wouldn't do it too soon.
Montesorri apps like Math City are also useful.
A number line that he can always refer to, just something put up wherever you do work, can help him develop an internal sense of number.
Numicon apparatus - like the number line track etc can also be useful.
Thinking Blocks website - based on Singapore maths style illustrations - is also v good
Talk out loud when you are doing any sort of mental maths - a lot of estimating, showing how numbers work., just to model things without adding pressure for him to do stuff.
Keep everything concrete and visual for as long as possible, and transition slowly to more abstract things. Much of the work at school might be abstract and written methods for things, but it's too easy to learn to do some of it by rote without understanding it, unless you also spend time doing the concrete work.