I have not dated anyone so young, but I have a 23 year old coworker (I turn 50 this month). He is so sharp, I sometimes forget his youth. However. I am well aware of the fact that he has so much yet to experience in life about EVERYTHING.
One evening, my young coworker randomly invited me over to watch a movie, stating he could never watch it with someone who didn't understand (the movie related to our line of work - financial industry). I honestly didn't know if that was true or if it was an excuse for some kind of booty call. Turned out his initial statement was genuine, and he just wanted someone to hang out with. (LOL! I didn't know if I was relieved or disappointed!)
Anyway, I had to get my head right in case... I think when you get involved with someone so young, you need to treat what they have to give as joy - but a joy that may be short-lived. They need to experience life, date around, may want to get married and have kids, may be poly themselves and keep you in their lives, but it's really not fair to them or to you to expect a long term partnership. (I am not saying it is impossible, just unlikely.)
Additionally, people that age are pretty wrapped up in themselves. Doesn't mean they don't want to be with you, just that whatever is going on right at the moment is going to have their total focus. At that age they really can't be wrapped up in what you need. In fact, he probably sees you as a confident in woman with a lot going on her life, and doesn't realize he needs to validate your relationship in any way.
As to your message to him...., I don't think it hurts to remind him about the time frame for the surgery (sorry you are having to deal with that, btw!), but maybe a more casual statement. "The thyroid surgery is weighing on me. What does your schedule look like over the next three weeks? I'd love to see you a few times before then." This totally skips the scheduling issue. He may give you a schedule, but you won't come off sounding like you want to control his schedule.