I have GAD (generalized anxiety disorder), and it's definitely less of an issue on Whole30. In general, any time I'm eating less sugar, my anxiety is lower -- it's a total vicious cycle, otherwise, because I (like many people) find food treats comforting, but they also cause anxiety spikes, so then I crave them more.
I'm rarely soap boxy about things, but if he gets nothing else from this: please tell him to cut out caffeine. I can occasionally have A cup of coffee, but I usually stick to decaf, because caffeine causes me to have anxiety attacks if I'm not really careful.
My general regime for dealing with my anxiety is this:
Work out 3-4 times a week. (I just do Zumba for 20 or 45 minutes, I really think anything helps, including walking).
Sleep 8 hours a night at regular times (I try to do 11-7). If I don't sleep at times that "normal" people sleep, I feel more removed and weird, so I try to stay on a schedule as if I have a regular 9-5 job, even though I don't.
Klonopin for anxiety attacks. I don't have as much constant anxiety as some people, I tend to have kind of spiked attacks that last a day or so. Klonopin knocks these out. I know it's often not the first thing doctors prescribe, because it has a high risk for abuse. For me, the daily pills (all SSRIs) did not really work as intended. It's better for me to treat things occasionally. Several of my friends with anxiety swear by various mood stabilizers; I think those are great for a lot of people, and a lot of people for whom SSRIs alone don't work alone find a mood stabilizer is great. Basically, it's worth it to spend time sorting out your meds, even though the process totally sucks.
Eat at regular intervals. Do not skip meals. Eat adequate meals. Avoid any foods that cause blood sugar spikes or crashes or anything else like that. I can go out for ice cream once a month, or have dessert on an important anniversary if I want to, but I can't/shouldn't stop at the pie shop on the way home just because it smells good -- it always smells good.
Interact with other people, even if it's just walking to the corner store for a coffee. If I spend too much time alone in my head, I get weird -- and this is even though I live with a spouse. I need to interact with strangers or people I don't know well. It's often really scary for me, but doing it more often makes it less scary, and it keeps me from obsessing over terrifying social scenarios, because I get a lot of regular feedback that I'm actually fine, I can talk to people.