I'm just going to summarize what i like from the mom's that did post and add that if you haven't done so already, you need to let the mom know that you don't think he's eating enough. Hopefully he will adjust to the transition soon!
First, I would definitely try to feed him in a quiet, dimly lit room - hard in the middle of your daycare, I'm sure, but some kids really don't eat with any distractions.
2nd, I would ask the mom to leave a couple of spare bottles and nipples or at least find out what kind they are and use those.
And third, find out how the mom does hold him, or what she sits him in when she feeds him.
Finally, I forget what the earliest stage for sippy cups is, but you could try one of those just for something completely different.
The only other thing I can think of is what kind of formula are you using, same as mom's? dry or canned? if it is a mix, what kind of water do you use, does mom use?
Good luck, feeding issues are so nerve wracking!