Hi,
My DS has a geographic tongue and although he is not a picky eater, some foods do aggravate him, like spicy or acidic food. Days when he's 'patchy' we have to avoid alot of fruits or fruit juices as they do tend to give him a stinging/burning sensation. I do believe it can also cause numbness occasionally so maybe that could put him off wanting to eat some days?
How old is your lo? can he tell you what sensations he's feeling?
It has been suggested that geographic tongue could be due to a Vitamin B deficiancy, although this is only speculated and doctors currently don't know for certain what causes it.