Has anyone gone through something like this? I'm really hoping it's a temporary thing, but thinking back it does seem like he's had a gradual decline and some personality changes over the last year, and it's only now that things have gotten quite apparent that I'm realizing there may have been earlier signs. I think the right thing to do is to talk to him and ask if he's doing okay, but we don't have regular one-on-ones and it will be hard to make this happen naturally. I don't think engaging HR is the right thing to do, and I don't want to go over his head to higher management either.
First of all, it's important to retain empathy and remain professional throughout everything. Unless you're concerned about his personal safety, it's not appropriate to try to diagnose his cognitive decline or push your superiors toward decisions. Focus on adjusting your own personal work behaviors to accomodate him while still getting your work done, as frustrating as that might sound.
Consider adjusting your interaction style with him to include more frequent updates. Perhaps a daily summary of what you did that day that leads with bullet points about what you're still waiting on (budget approvals, decision making). Send him a short e-mail summary about conclusions from your conversations with him. These habits are actually good to have anywhere, but they're especially helpful in this case.
Second, don't assume that you're the only one seeing this. Your manager's manager is almost certainly aware of what's going on. If your boss' boss starts interacting with you more, stay professional and stick to the facts about work rather than making it personal about your boss' problems. Your superiors will put the facts together and come up with a process in due time.
Finally, recognize that your work situation could change in the coming months or years and prepare yourself for where you want to go. If your boss retires, do you want to move up into his position? If so, now is a good time to ask to take on more responsibilities and offer to help him out where possible. If not, consider where you'd like to go next. Would you like to be on the same time with a different manager? Then maybe stick around. Are you on the fence about leaving already, and this is the last straw? If so, maybe don't delay and start finding a new job.
If this becomes too much of a problem for your own career or job satisfaction, it's better to switch teams or jobs than to try to handle this situation directly. It's a difficult situation for everyone involved, but the last thing you want to do is end up on the wrong side of a difficult personal/medical situation.
People behave like this when they don't feel involved and don't care.
They forget things things, like why decisions have been made, because they are not paying attention.
Sometimes it's passive aggressive; someone thinks a bad decision was made, and so keeps pretending it wasn't made. Every time it comes up, it's "why are we doing this" all over again; which really means "why the hell are we doing this idiotic thing" not "I genuinely forgot, someone remind me".
It's also possible that they are juggling more plates than usual because of something else happening elsewhere that you aren't aware of in the grand scheme of things.
Reach out and help by trying to stabilize things as best you can to make interactions with you more predictable and routine, and pay attention to making their context shifts easier. Be a subject area cache for them. Or if you have a really good working relationship, ask what's going on or if there is something you can help with. If they can't say or it gets awkward, it may be a personal sphere change in which case proceed delicately and just do what you can until they get their feet back under them. If you think plumbing through code or analysis of something static like figures is hard, imagine all the problems you're working on suddenly getting soft, squishy, and ill-defined with terribly documented dependencies that need to be constantly excavated by someone, and can change on a dime given the winds of politics in an organization. That's management. Trying to keep a foot in the technical side of things will hampen their ability to do the tough squishy social process forming required to facilitate smooth organizational communication, coordination, and change.
Science is easy. Engineering also. Getting people all on the same page? There's really no substitute for having tried to do it.