2 yr olds know... thats why they are pushing the limits in the first place.

I have a 3 yr old & 5 yr old & they are made to pick up 10 toys each. They make a game of counting them as they go. Maybe you could try something like that.
Ask him if he would like to play a counting game with you when its time to pack up. You pick up 5 & he picks up 5 counting as you go. Make it fun to tidy up.