C.S.
I have to say that he sounds completely normal to me. Many of the things he is going through would start happening at this age irregardless of whether or not his daycare positioning had changed. I'm sure the 2 year old is influencing him to some degree, but he was with her before so it's not that drastic.
What is dramatically changing at this age is his communication level. Vanae was right, he realizes now that those around him are communicating and he wants to also. I've worked in the 1-2 year old room in my church nursery for 3 years now and I know how frustrating it can be when the parents don't teach their child any communication and that's from the adult side, I can only imagine how frustrating it is for the child. I would do Baby Sign and Speaking Hand in Hand. It's what we practice in the nursery and it's quite effective. Some children don't really grasp talking until the age of 2 or older, but they can do some of the simple signs to get their point across, which is way better than pointing and grunting! Please, Thank You, Cracker/Snack, Drink, all of these are incredibly helpful.
I would also highly encourage that if you want a well-mannered child to start it now, when they are first learning, and don't wait. If you speak to them with Please and Thank You they will also learn to say it themselves. Once they grasp it you can lightly encourage it (you can't have more crackers until you say please... ). It may sound harsh, but they won't starve to death and quickly get the point that it's required.
Finally I agree with getting him on a regular eating habit. He should have 3 meals and 2-3 snacks during the day, plus fitting in an occassional bottle. If he's started not wanting his bottle you might start transitioning him over to a sippy cup or even braving a regular cup. More and more they are saying sippy's are bad for their teeth so you make that brave call for your own household :)
I'm sorry your sweet little boy is gone for now, but eventually he'll be back. I went through the same thing with my son, who is almost 5 now, and while not all the sweetness is back (they are boys after all), he's still a very affectionate nice little man.