I don't know if this is something you can talk about with your nephew, but I found trusting myself and taking advice (medical, but also often anecdotal!) all with a grain of salt, to be super helpful in getting better.
There's actually still so much science doesn't know about treating anxiety and drepression....even exactly how antidepressants work! I went on them for a little while for GAD, and while I think they got me through a rough patch, they weren't the solution forever, except doctors weren't giving me the solution forever, I had to find that out for myself.
I think doing something like whole30 can be really empowering because you physically feel the effects of treating your body with respect...its a pretty powerful feeling to wake up and go 'holy shit, I haven't feel anxious all week...maybe this is awesome?!' Especially if he's got some one there to help him like you (and you let him lead how much help is given/taken.)
One personal point is, as some one who also used to suffer an eating disorder (I guess this could be relevant to anyone who gets obssessive behaviours)...I'd tried googling the negatives of whole 30 and the ONLY thing I could come up with is that it makes some people obsessive about food. So at the start I told myself I was going to eat whole30 style, but not beat myself up about finishing things off in my cupboard, or obsess over restaurant menus....but I planned lots of meals well, and then found myself being compliant really easily! So it turned into proper whole30 without any obsessing at all! And now I feel so good sugar cravings (the only real craving I still get) are easy to beat, because I don't want to feel shit, it's as simple as that.