You do pretty much need to not buy cook-chill products anymore, with just the occasional exception (such as one type of Co-op fishcake being labelled GF).
Oven chips are my bete-noire. They simply do not require the addition of wheatflour, but they almost always do; occasionally the very cheapest ones are OK and Aunt Bessie's are, but McCains are very heavily promoting/aggressively marketing so that the most likely bags in the small supermarkets are always going to be their frankly, poisonous, offerings instead.
Birdseye Potato Waffles are OK. Their GF nuggets and chicken/fish things are rank, dry things that scrape the lining off your mouth, though.
Instead of soy sauce, you are looking for Tamari. Can't have shop bought sushi though, even if you don't use the soy fish, as it's likely to have been cross contaminated (as I found out to my cost). And those marinaded fish/chicken/pork things are almost all out of the question now, as they'll have soy sauce including wheat in them.
Mustard - French's is OK, Colman's isn't unless you buy the square tin of powder and mix it up as you go along.
If it's made by Walker's, it can't be eaten. Some Tyrrell's are OK, but not all of them.
Little Moons are great and GF if he likes ice cream/mochi. Whilst the posher ice creams are more likely to have something he can eat, almost none of the dairy free ones are safe.
GF pasta is actually really nice. Cook it in lots of water - Garofaolo is the commonest brand and is fine, De Cecco is nicer.
Noodles are generally off limits. Pure buckwheat, despite the name, Soba noodles are OK - Clearspring do them, but most have wheat added - no egg, udon or unspecified noodles. Rice noodles are possible and just need soaking in hot water for a brief period before adding to a stir fry. Most stir fry sauces, some gochugang and miso are dubious - check them very carefully.
You can get a hell of a lot of flavour from other sources rather than jars - chilli and garlic infused olive oil, adding olives to stews for a rich flavour, for example. Worcestershire sauce is out unless specifically marked GF.
Watch out for even dry rice and pulses - some of those are labelled because they can be contaminated during packaging/processing.
He might also have issues with lactose because of celiac damage to the part of the gut that produces the enzyme to digest it. Lactofree milk, Lactaid tablets for the essential ice cream, but he may be OK with hard cheeses or goat/sheep milk ones. This can recover to some extent.
Basically, you will need to read every label. Every time. Separate butter/peanut butter/jam if you continue to eat standard gluten foods, separate spoons, thoroughly scrubbing every pan between uses, separate chopping boards. And don't be surprised if at some point he develops an almost childlike obsession with something if you find a GF version - after months or years without something, to find it again can get you ridiculously excited.
I still get caught sometimes, but that's generally from an undeclared cross contamination.