don't want to add pressure at home
You could try Khan Academy, just to see if he likes it and stop if he does not. However, as they collect "badges" for certain tasks, dd used to quite like doing it.
Or play games which will teach him; shops, junior scrabble, etc.
Or make up games. "What shape am I?": One person thinks of a shape, (2d or 3d), the rest have to guess it from yes or no answers.
Get a roll of wall paper and put a large numberline on the back of it. You can then do sums, including into negative numbers, by your child walking up and down it. Or they can draw trees at say multiples of three and houses at multiples of 4.
Have pizza parties, with drawn and cut out pizzas, where toys want different fractions of the pizza. If one has two quarters, that is the same as a half, etc.
For teaching maths, keep it fun, make it colourful and stop when the child wants to stop.