Question: When he brings up ideas and upon further reflection I see barriers or limits (lack of funds/resources, etc.) do I bring them up, or do I just listen and let him continue to brainstorm? These limits/barriers do exist in reality (believe me, I have gone around and around to reduce them or eliminate them but they do exist) but I don't want to interrupt his process.
both.
ok, this is what it sounds like:
when he brainstorms things with obvious problems, he probably knows they are there (at the very least he sounds very conscious about his lack of funds).
so he's is essentially brainstorming IF fantasies - "if i had X i could do Y". what he's doing right now - brainstorming the problem itself - only works when the solution is inherit to the problem... which isn't always the case. instead it just leads him to brainstorm if the problems wasn't there.
which makes perfect sense:
he is thinking about "the goal" (solving a problem) so he is thinking what he could do "with the goal" (what he could do if the problem was solved).
Ne doesn't start from a target - it doesn't start from the problem its trying to fix
rather: it starts from potentials of resources - what could i do with X, what can it be changed into.
you can think of it reverse Se: instead of asking what something is, Ne asks what it isn't [yet] and how that can be changed.
and you brainstorm what you could do with X until it strikes Y, and you brain storm what you can do with Z until it strikes X, and if your lucky it eventually strikes ground, "but wait, i can actually go and do Z...." and when those connections are made, that's when it gets exciting.
think about it like evolution - water-wave-creators (fins) can be seen as pretty useless out of the water, but evolution explored what amphibians can do with them and found out they make for great ground pushers (legs), which happened to work better with toes, which turn out to be really great at grasping on branches, which turned out to be not-so-horrible tool manipulators, who are turning out to be pretty decent in nuclear bomb construction, which then made the land uninhabitable after ww3 and thus made the whole move out of the ocean into a really bad decision (stupid fish). the point is: NTP brains aren't that far off (with Ne generating mutations & cross breeds, and Ti playing the role of the selection of the fittest.. or at least most logically-consistent).
so how about this: you don't brainstorm the problem, you seed the problem at the back of his head, and instead you brainstorm about everything else.
leave the problems at the back of his head, and encourage the possibilities to accumulate until they hit the problem.
on the immediate level, accumulating possibilities and letting them connect carries the same experience of a cathartic moment, the Ne equivalent of "ahuh!", which is probably the best way to invigorate him on a moment to moment level (other then fucking him), and help him regain his self confidence. in the long term, as random as it might seem, it can prove to be a remarkably innovative problem solving mechanism - and could actually help him figure out the footholds for climbing out of the rut.
so back to your question:
here's what do you do...
1. let the brain storm continue
2. take mental note of the problems
(keep in mind that the problem with the possibility doesn't make it irrelevant, it just makes it into a potential later step)
3. mention them later in a btw way... and occasionally while he brainstorms something else completely.
result: if the problem of the plan to do X was needing Y, he'll have Y at the back of his head when he thinks of Z, and... he might gain some of his mojo in the process.
on a side note: since not treating terminal diseases is sort of.... you know, very close to suicide... there seems to be a very basic very convincing argument against that - if you continue to live you can always change your mind in the future, if you die changing your mind will be a bit difficult, and since you can never know what reason you might have to live tomorrow that you haven't figured out yet (i.e. writing his book before thinking about writing the book).
i'm exhausted so i am hoping i was able to make it make sense, and maybe even useful.
(if it didn't it's tomorrow-me's fault, and i am sure he'll take full responsibility. i had nothing to do with it).