Pumping will definitely get less, which is a bummer. A better, stronger pump can get more, though. That being said, if you are interested in trying to increase your supply, you could supplement. I take GoLacta, the highest dose. It definitely helped when my oversupply took a nosedive around 6 months when my son got sick and I didn't pump to compensate when he wasn't eating as much. It just never recovered on its own. He is now 11 months and i am still using it and plan to use it at least 1 more month. He won't take any formula either--well rarely. I would try different formulas because they all taste different. I know that is expensive, but if you're concerned about the nutrition, then it might be worth a few more. Try dairy and soy versions.
When I weaned my daughter, my lactation consultant recommended oat milk...it's very sweet and tastes like the milk after a bowl of cheerios. As mentioned by others, breast milk is super sweet, so it might be an option. You can find it in the boxed milk section at health food stores.
My daughter didn't care at all when I finally gave up pumping 4x/day to keep my supply up at 14 months. She just moved on. I'm not sure how my son will do. He seems to be telling me that he's not interested in stopping. At least he has outgrown his dairy and soy sensitivity. I was about to go crazy!! Good luck. Oh, my pediatrician said that babies this age should be getting a minimum of 16oz of formula or breastmilk a day. So I would at least keep him hydrated with water if you're not getting that much. Baby's Only organic dairy formula has a vanilla-y taste and is very sweet. You can order it online or get it at whole foods/health food store. The soy one is kind of weird, but I have read that breastfeeding babies like it. (I supplemented my daughter with it at 11 months, but my son doesn't care for it.)
Hope things get better!