It's been my experience that I am MUCH healthier without grains, vegetable oils, and legumes in my diet, regardless of the reality that there are SOME things that are still health issues for me. Frankly, if I'm debilitated by my allergic reactions, and cutting some things out of my diet means that I'm at least functional and not having quite as MANY reactions, the few that are left give me less issues.
For many people, they come to this way of eating to try to resolve very visible or noticeable issues -- when those resolve, the underlying issues that may not have been as noticeable before become much more apparent, so then the push is on to fix every little thing. I'm not sure this is necessarily a healthy approach, from a mental/emotional PoV. See, I think it instills this idea that, for ancestral eating to be of any value, it MUST be a cure for every ill and discomfort. Reality is that we are only as healthy as we're born, minus whatever permanent damage we've already done -- so expecting perfection is an unrealistic expectation for most folk... and definitely so for someone like me, who came to this late in life with a LOT of residual damage.
Then there is the issue of quality-of-life. For some folk, whether or not they might find some relief in ancestral eating, and the reality that eating fresh, unprocessed foods that do not contain processed vegetable oils, grains, or legumes would be beneficial, from a health perspective, for anyone with human genetics, actually DOING the diet would set of feelings of extreme deprivation for some people which wouldn't help their FEELINGS of well-being regardless of how good the diet was for them.
My suggestion would be to encourage your companion to try new foods and eat fresh foods as much as possible. Try making fresh, local foods whenever you share a meal -- let him learn at his own pace. Show alternatives to things that require grains, legumes, and veggie oils, and let him absorb things according to a timetable that is sustainable for him. Let him see by YOUR changes what he could do if he made the changes himself. That's helped me to bring a few truly resistant friends and family members into a better way of eating/living... and I remind myself all the time of how long it took ME to take this in, and how many baby steps I took along the way, which helps me to remember to be encouraging and not judgmental.