I can't give you any TX specific advice, but for the ADHD related depression and anxiety I can definitely give some advice.
First, CBD oil may be a suitable option. Some is pure enough to not show on a THC test as positive, so that may help. CBD is the part of weed that gives most of the mellow and chill, so it may be enough for him.
Second, depending on his ADHD presentation he may benefit from hard exercise. I find that 5x5 weight lifting is really good, along with some sort of cardio if possible. Lifting heavy things takes a lot of the hyperactive energy away and leaves me clearer.
Third, non stimulant meds. Some people find benefit from things like modafinil and if he could get on that it may provide enough support to quit the weed for long enough to get onto a stimulant.
That said, the USA has really intense laws about stimulant meds as well as a bunch of strange stuff about testing that seems punative. I have meds here in Australia and I have not taken a single test. I know there are online clinics that do ADHD treatment and also there are less than legal methods of getting stimulant meds. I know a couple of people who tried stimulants before getting them prescribed. Maybe that would work for his situation.
As for efficacy, I tried a whole bunch of anti depressant meds, anti anxiety meds, all sorts of stuff, and honestly nothing worked. My k10 was absolutely abysmal and I couldn't really work. Stimulant meds mean I can work, support my partner, engage in things I enjoy, and live life quite well.
I had to come off my meds for a couple of months while going through a heart valve replacement (unrelated to the meds) and wow, I had forgotten how bad it was without them. They are great, getting the dose and specific stimulant right is important, but yeah, worth every bit of effort. Absolutely life changing.
It is more than that. In previous studies from the authors they have controlled for the nutritional content and the processing and found that the content itself, so carbs fats etc, have a certain amount of causal influence on health, but the processing also has a separate and significant effect. Just having less processing seems to have a meaningful effect. This means less of the additives like milk powder, xanthan gum, sweeteners, flavourings, extracts, and so on. The exact mechanism seems to vary depending on the specific case, but separating components of food and then remixing them as well as adding non food components and processing with heat and pressure seems to make these things no longer digestible and safe.