Is he taking both sides at each daytime feed? We didn't drop the DF in fact until about 9mos, I think for both LOs (bit hazy) so perhaps that is playing against you (don't think you're DFing?). Counted that way, they were both still having one NF at this stage!
It could be a coincidence that he only did two NFs after you added in a top-up, but I would stick with that and with the strategy of trying to increase his daytime intake of milk. Clearly he's keen to have the milk, and he's young to be relying on solids for nutrition.