do you play on regular days?? could you tell him when to set the next couple alarms for??
if not, tell him "you can ask us when we're playing again next time, and set it in your phone. I am autistic too, and if I can manage to set a reminder, so can you. If you can't manage to set an alarm on your cell phone, maybe you aren't high functioning enough for our crew. we like you, but you've gotta be a big boy about this one, cause we can't hold your hand the ENTIRE time.."
be kind, but be firm. encourage him to be more self sufficient, ya know?