1 to 1.5 litres of water per day. I saw a thread about this not long ago too and thought I would check it out for certain and it is at least 1 litre a day.

You can try reward charts, games where you race one another to drink , plain water icy poles, special cups, special water jugs, having him sip when you sip, what else? Special treats if he gets through so much water. Not pouring it all at once so it doesn't feel like he can't reach the goal so little goals each time.

I think that is most of what we tried. ..didn't work for us we're still stuck at 600 mls much to our dietitians dismay lol.