First of all, congrats on nursing your son for 10 good months and more! I nursed my first for 17 months and am currently nursing a 22-month old.
I think my 10 month olds nursed at least 4 times a day, probably more like 6-8 (sometimes for just a short while). They did not take a bottle and did not drink well from a cup yet, so I was careful to offer the breast lots so that they got plenty of liquids. If you are worried about your son's diaper output, I'd just offer the breast a couple extra times a day and a sippy cup of water or diluted juice other times. If he's been too busy to take a drink, your offering might remind him that he is thirsty after all. Here's a site with lots of good info on nursing older babies and toddlers:
I was hoping you wouldn't get too many responses urging you to wean, but I see that there is one already. There are so many reasons to keep breastfeeding to a year and beyond, so please don't let our bottlefeeding culture dictate your decisions. (BTW,10 month olds do not "self wean" without subtle or not-so-subtle pushes from mom, so if that's not your goal, please know that you're not forcing him). Do what feels right for you and your son and know that for as long as he nurses, he continues to benefit. (My first never had an ear infection until I weaned her-- then promptly got 3 in the next 6 months :( ) More on that:
Anyhoo... my guess is he's a busy little guy who might be forgetting to sit down for a nice nursing session or to ask for a drink now and again in the midst of exploring his world. Keep offering the breast and the cup, and I'm sure he'll get what he needs. :)