> I am an admin assistant who is co-chair on a committee
> along with an assistant prof who is also a co-chair.
So, ah, pardon a delicate question but 'admin assistant' to whom/what? If you're admin assistant to someone like an assistant prof or around that level, that's one thing. If you're admin assistant to the college's Dean, that's quite something else.
> I have said that I can help out with his events if needed and expect the same in return.
> ... he asked me to handle the scheduling. ... I received another email today to please craft an email (on one of his initiatives) to send out to our members something related to one of his initiatives.
> But now I feel like I am being mistaken for his own administrative assistant,
You've not mentioned all the times that you've asked for his help on this or that portion of your events that you're handling. Now, if there have been no instances of those, and you're just sitting around doing your stuff, you've openly declared that he's "very busy" (way more than you, apparently), while being receptive to his requests for help on his stuff. Well, I think you will get what you bargained for (acting as his assistant for his portion of this co-chairmanship).
If this were the case, while technically you may not need any of his help with your events here, tactically and politically, I think you may be wise to aggressively enlist his aid at a level that at least offsets the amount of help that you've been giving him.
editted to add: You're sounding a bit like the sort who would need a 'reason' to stop assisting him. Well, if he declines to fulfill your like requests for assistance on your events, surely that's reason enough to no longer be so helpful with his (imo).