A little late, and I know that you're not mainly looking for apps, but if you do want a good app (perhaps when he outgrows/gets bored of Numberblocks, which btw I also think are excellent, an amazing resource) -
look at the Dragonbox ones ('We want to know'). I am surprised they haven't been mentioned yet!
For your child the one called 'Numbers' sounds quite appropriate. It is fantastic for developing 'number sense' - second only to doing the same with actual concrete objects, IMO. And young children who love numbers usually love this app.
My youngest is now five and in reception and massively enjoying 'Big Numbers' (which teaches column addition). Eldest is eight and in Y4 and can do column addition in his sleep, but still enjoys that game, just for the fun of it.
At age 4-5 my eldest enjoyed the 'Algebra 5+' and now he is just about to complete the 'Algebra 12+'. These apps are so good, he literally does algebra for fun. And did so at age 4 and 5 as well.
They also have a geometry app called 'Elements' which DD played around with a bit at age 3, but wasn't really 'getting it'; DS at 8 has picked up quite a bit of geometry through this game. So perhaps best for slightly older kids.
And they have a chess app 'Magnus Kingdom of Chess' which I'd thoroughly recommend to anyone 4-8 or so, who wants to learn the basic rules of chess. We use chess as 'sideways' extension, for our children to learn to problem solve, deal with frustration and defeat, and intellectual challenge. But before you reach that stage where chess can be used for that, you need to get to the point where recognising the 'legal' chess moves becomes automatic, and for most young children (4-6) this is hard to achieve, unless they are a prodigy of some sort. But this app enables development of a very good 'feel' for chess moves, just by playing a fun game. ('Older' children 6-8 can still benefit from the app, but there are other ways for them to learn as well.)
So in age procession, start with Numbers, continue to Algebra 5+ and Magnus Kingdom of Chess from age 4 onwards, then Big Numbers from age 5, then Elements and Algebra 12+ from age 7 or 8. A fantastic family of apps. (And no I don't work for them!)
There is also Osmo, which are apps with concrete hands-on elements. In Osmo 'Numbers', you lay out (real life, concrete) number tiles or tiles with a number of dots on them, to create new numbers in various ways. The camera sees what you are doing and translates it to the screen. Quite good IMO but terribly expensive. We would never have bought them but got them as a gift.
They also have coding and story telling and tangram apps (which are also good), and more I think (I'm not up to date).