It’s said that the best apology is changed behavior. I would say just let your old apology stand and make sure you’ve changed the things he was upset about. Give him his space regularly and allow for some time to go by between replies without reading into things and that should help a lot.

If you do feel you still need to apologize, just give a short, quick apology when you hang out. It sounds like he was a little overwhelmed by maybe your need for attention? So a long apology might be taken the wrong way or feel like boundaries are being crossed again. So I’d keep it short and simple. “Sorry about all that, I’m glad we’re still friends.”

If he does cancel, try not to read too much into it. Things really do come up sometimes, especially if he’s now sick with the flu.