There could be a lot more to it. Is he on metformin or any other glucose controlling medication? If he is drastically changing his diet, then his meds need to be adjusted as well and he needs to work with his doc on a lot of this.
My own personal experience is that any wheat at all throws me off. Certainly, a 90% decrease is better than nothing, but even a little bit could be throwing him off. Also, don't just look for the wheat, but look for any processed foods because those very often have boat loads of hidden sugars. I was looking at "healthy" granola bars at Sam's Club this morning and 4 of the ingredients were a type of sugar. I think I'll get the ingredients to make the energy bars from the recipe here instead.
Your family member is certainly on the right track and this isn't a quick fix by any means. I think your main goal may just to keep encouraging him to keep up with it and not give up when he doesn't see results immediately. Good luck!