I'm in a very similar place as you, but I'm doing this for myself. I knew I had dairy issues, so that one was obvious. I read gluten problems can go hand in hand with dairy, so I went GF, too. After a few days, I realized that the GF corn products were also giving me issues, so corn went, too. I have accidentally exposed myself to dairy a few times, because dh does the shopping and is rather confused, poor thing. Then I forget to recheck every label after him. So, after less than a week it was very clear that both casein and lactose cause me reactions. But, I'm not sure a child would be able to verbalize it. Casein causes a nearly immediate reaction, with my ears and sinuses swelling up and pain shoots down my lymph system in the back of my neck. Lactose is takes longer, but it saps my energy, and I get a milder version of the symptoms of the casein. I can barely get myself out of bed the two days after a lactose exposure. It takes me about three days to clear up enough to be 'normal'. I don't think I've yet hit a month of no exposure, so I can't say what that would feel like. On the other hand, corn gives me a feeling like my intestines are glued shut. Uncomfortable, but not painful. I get constipation with it.
I started this because I finally got sick and tired of living with IBS. It took an entire week to start having regular BMs, probably because it took almost that long to figure out the corn.
I know different people react differently, but I hope when you reintroduce, my experiences can help you ask your child specific questions about what he might be feeling.