I babysat a friend's child who did this same thing. She would nurse him in the morning, then he would hardly drink a thing all day from a bottle, then nurse like crazy when mom got home in the evenings.
I kept him on a good routine and he eventually started drinking more from the bottle, but not much. He would drink a couple ounces in the morning and a couple in the afternoon. He was growing and learning normally, so he was obviously getting enough to eat. Also, we learned that he preferred room temperature milk, rather than warmed milk.
If you can feed him yourself in the middle of the day, that's a MUCH better option than pumping anyway, so I'd do it!
If not, I think you should definitely keep pumping if you can, and find out more about milk storage. Fresh breastmilk is safely stored in the refrigerator for 2 or 3 days. You can freeze is for 3 months, but once it's defrosted you need to use it within 24 hours. You don't have to waste as much if you are careful about how you store it and how much you store. If you're freezing what you pump, you might think about freezing just one ounce at a time. That way you just defrost a little bit. Defrost a couple ounces, then leave bottles of fresh milk that you pumped the previous day in case he suddenly decides to drink a lot. If he doesn't drink the fresh milk after 2 days and it has been continually refrigerated, freeze it.
And, keep it in mind that he will not starve himself. If he's hungry enough, he'll take what's offered (just make sure the daycare workers are, indeed, making an effort to offer it at regular times!). If he's got the stamina to hold out for mommy, so be it :-).