HELP??

I have a feeling my husband has a food intolerance/allergy and would like to try and figure out what it is.

I read on here somewhere that it is likely that foods you often crave are usually those you are intolerant to.

He gets headaches, stomach aches, general feeling of not well, every once in a while (once every week/2 weeks etc) and haven't been able to find a conclusive link when I ask him what he's eaten that day.

So I was thinking of trying to put him on a diet that I set out & track how he feels but I dont know what to include, not to include to get started. I dont even know what categories to lay out to start eliminating.

Does anyone have any ideas? or good links? Thanks!