It sure doesn't sound to me that you did anything wrong at all.It sounds like you did something nice,which was telling him to have a great time in college.I see no reason for you to call it a mistake or to even apologize.

And,since you did already apologize,there's really nothing more you should do.If you were unfriended for something that petty it says so much about him and this is his issue,not yours.

For real,I don't think this was anything 'bad', wasn't a 'bad decision' or even a 'mistake' you made .I have done the same,even when I wasn't buzzed.And it's not your fault or your problem how he reacted.