I agree that it seems like a steep drop. Without knowing how much solids you are feeding him, it is kind of hard to say if the solids are the absolute cause of his cutting back on the bottle. You may also want to consider the possibility that the drop in formula has nothing to do with the solids at all, but could be related to the bottle itself (needing a faster flow teat, temperature different etc), or another unrelated issue like teething or illness. Any of that going on?