I have a seven month old in my daycare and she does the same exact thing. We have chalked it up to be that she is too busy playing to eat. Her mom started sending baby food and that has helped tremendously. I also give her a cup with a little water or formula when we all sit at the table for lunch. She really wants to be like the other children and so will eat and drink when they do. She usually has a handful of cheerios at the table as well. She usually only has one six oz bottle a day, maybe.
Could it be that he is too busy playing that he doesn't want to take the time to eat? Is he crying and then won't take a bottle? Is she actually sitting down and relaxing herself when she feeds him? Is she frustrated and he is sensing that? Does she prop the bottle or try to make him hold it himself? These are all questions I would ask her. There have been many times the baby wouldn't take a bottle from me when I was already having a bad day and she sensed those bottled up feelings from me and once I would walk away for a few minutes and come back relaxed she took the bottle.
Probe your sitter a little more and maybe just focus on him eating only the cereal and baby food. Have her try a cup and see if that works. I give the baby the same type of cup that all the other children have and she has no problems. I wish you the best of luck in this situation and I say as long as he is happy during the day with the sitter just pass on the bottle(still send one) and give him the food.