My 10 year old can go all day sometimes without drinking and other days he drinks over a litre in one sitting. is that cause for concern?

Dehydration: you can tell if his urine is very dark or have strong smell that your son is not adequately hydrated, unless he gets his fluids in other forms (juices, sodas, etc). in either case that is not good and not healthy on the long term (kidney problems, mineral deficiencies). try to talk to him on why it is importance and create a log. minimum intake should be around 1.5 liter per day.