The first thing I would say is to give him puzzle type questions (which seem more fun ) rather than pages of sums (which may seem like a punishment). I'll think of some resources such as number based 'magic tricks'.
Simple counting games are good to- but get him to count in 10s or 20s, then try 5s then 2s.
Play games like snakes and ladders, and perhaps even monopoly,
There are some sites that will help such as mymaths and if you want to try worksheets (though I think it may be too much as this stage) I can suggest some.