Could just be a thing he does and a few good gags will teach him in time!! He could though just be a little bit on the low registration or less hypersensitive side- you know some kids only need a small amount of food in their mouth to taste it strongly and feel that their mouth is 'full' and others need more bulk in there to get a sense of the food? Not a problem at all- just natural variation! Perhaps if you up the flavouring in the foods- add some spices, herbs, lemon juice, make it colder or slightly warmer to give it some more 'punch' so that he knows something is inthere. He may actually enjoy the food more too if hes getting more out of it! I of course mean to do this to the appropriate foods- some people like stronger flavours. I know i like heaps of garlic etc- whereas DH can taste the tiniest amount...