Welcome Mithawk! My immediate reaction is that it probably isn't possible to avoid lapses, and that the most productive place to focus may actually be on the reaction to a lapse - how to avoid letting one affect the next move and the next game. Chess is very unusual, compared to other things that one might think of as using some of the same skills, such as mathematics, because of the real-time competition aspect. Maybe it's more like music performance, or more like fencing, in its need for utterly lapse-free concentration, but of course it's unlike them in the kinds of skills it uses. With the latter group in mind, I actually wonder whether Alexander technique might be helpful? That might sound like a weird idea, but if there's a good teacher of that near you I'd suggest having a conversation about it. There's something about the way it encourages presence in the moment that might be appropriate (and it could hardly hurt, especially for someone who needs to be encouraged to go out and probably spends too much time sitting?).
Are there specific things that he could check, routinely, before committing to a move? Maybe a literal memorised checklist might be a good prompt; although I suspect that the number of things that would have to be on it might make that impractical.
