Gonna add my upvote for this too.

I actually have a horrible memory. I'm working on it in any way I can, but meanwhile I deal with it by making constant reminders/appointments in my calendar on my phone, and having each of those appointments have before- or on-the-date reminder notifications. I do this because I got well and truly sick of just forgetting things all the time.

My wife still reminds me for things, of course, but if it's a big/important thing I'll note the bloody thing in my calendar, with reminders.

My point in all this is: If you do that reminder one last time, and also let him know that you're not going to remind him again, at that point you've done all you can and you're no longer the asshole. You can't be his brain. He's got to take responsibility to remember to take care of his own stuff, just like I've realised I need to.

Me? If I do all this and still manage to forget something important, and my wife reminds me about it on the day or after, I do get annoyed. But not at her - at myself for somehow managing to miss putting it into my calendar. So, if he lets it lapse and gets mad at you, you've got to just keep in mind that it's not you, it's him.

tl;dr: Consider this a long-winded comment of me just agreeing with what's already been said and what you've already said you'll do... Oh, and best of luck. :)