Well, he's going to know that he hasn't been well, isn't he?
It's great news, the doctors have found out exactly what's causing it and all you have to do is avoid having it and he'll feel much better.
It's gluten, something that's found in wheat, barley, rye and a lot of preprepared foods/things in packets, so it means having to change some meals a bit (and everybody in the family will change so there's no chance of accidentally getting some in his food - it's easier to be wholly gluten free than it is to prevent cross contamination or watch somebody else eating what you want in front of you).
GF pasta is really easy to use - the Garafaolo brand is almost indistinguishable from normal pasta in texture, taste and cooking. Pizzas are less good, but still absolutely edible - and making your own is the easiest way to ensure safety and getting them as near to how he likes them as possible. Corn wraps are nicer than wheatflour ones anyhow. The M&S Free Froms are pretty good.
Baking means learning different ways to make things to avoid the gluten in normal flour.
As he already likes baking/cooking, you've got something he can become an expert in.
It'll be annoying about the no MaccyDs or birthday cake that isn't specifically GF, but that can be got around.
And no, there is never a day off. The first time he gets glutened once he's stopped being poisoned on a daily basis will make it clear that he can't 'have a day off' being Coeliac.