Has he used anything to drink from other than a bottle? If not, maybe start with a Nuby, which has a soft nipple like spout. I know it sounds backwards and maybe not appropriate for his age. But the soft spout is easy for them to get liquid from, and it sort of feels like a bottle. Both my kids started with the Nuby, then slowly transitioned to a hard spout and finally a straw cup (well 17 month old DD is not quite there yet). DD now drinks milk mainly from one of those Take n Toss sippy cups, but still uses a Nuby (which she LOVES) for iced water on weekends.
I know it's hard to see them so thirsty after daycare, but both my kids are this way too. Have you tried the sippy cup at this time, when he's thirstiest? I know my DD will easily chug down 4 oz of milk in her sippy in a couple of minutes when we first get home from daycare. Also jessmum46 is right, there is a lot of liquids in food, especially if your LO likes fruits and yogurt like my DD does. DD's pediatrician also told me that they don't really need a lot of liquid, maybe 16 oz, which doesn't seem like alot does it?