Don't make him do too much, please! He needs time to relax properly as well.
But there are lots of ways of making maths fun. Websites - BBC bites I've, as mentioned above, also try go ogling 'numeracy boot camp'. The nrich website has a lot of mathematical challenges which can be quite motivating for some.
Times tables - try putting the times tables to actions, the macarena dance works quite well! Show him a whole load of numbers all jumbled up (all the times table answers, plus a few more random ones like 130, 250, 46 and decimals like 2.4). He then has 3 minutes to cross off as many answers as he can by telling you an appropriate number sentence e.g '0.7 x 9 = 6.3' so you cross off 6.3. Do it regularly and try to improve his score. You can vary it by asking him to start from the number given and tell you a division.
If he's working at a high 4 or 5, you could also give him a times table code cracking challenge. You write out a times table (start with an easy one like 5x) but substitute a letter for each digit, and put them in a different order. He then has to cracking the code and find out what each number stands for.
Reasoning - important at level 4 and especially level 5. Give him a statement (e.g 'Multiples of 8 are also multiples of 4'. He has to tell you if it's always, sometimes or never true.
I'm sure I can think of some more, but don't want to make this too much of an essay. Let me know if there's any specific areas he needs to work on, and I'll see what I can come up with.