Thank you for all your replies and suggestions!
Here's what he typically eats during the day:
6:30 - wu, BF
8:00 - 3oz oatmeal, 3oz yogurt, both with fruit puree mixed in, 2oz pureed prunes
11:00 - wu from nap, BF
1:00 - 4oz toddler meal, 2oz steamed veggie, 1/2 pear
3:30 - wu from nap, BF
4:30 - crackers, sometimes more pear
6:00 - 2oz quinoa mixed w/ 4oz toddler meal, 2oz steamed veggie, 2oz pureed prunes
8:00 - BF before BT
He's always had a wonderful appetite. He just started to eat "real" food a few weeks ago. Until then, he would not eat anything that wasn't pureed. We're trying to slowly phase out the toddler meals, but so far he hasn't taken to eating meat. He is prone to constipation, so we give him prunes every day. He does get calcium from yogurt and calcium-rich veggies, like spinach and green beans.
As you can see, the only fluids he gets the entire day is from BF. I'm just worried that he's not getting enough fluids, but he does have plenty of wet diapers.
We have started giving him a straw cup filled with water at every meal, but so far he only puts the straw in his mouth to bite it...he hasn't tried or figured out how to suck the water out. We have also tried using a free-flow sippy cup, but as soon as we put the spout in his mouth and tip the cup up so he gets a little water, he gets upset and pushes the cup away. I'll try to sit him on my knee a couple times a day with a sippy, maybe as soon as he wakes up, like you suggest, Kara. And maybe I'll even leave a few no-spill sippies around the house (although he'll probably just play with them).