Depends if you have things you want to do. Personally I like to know what is happening so I can organise my life. I don't know how well you know him.
e.g. Tomorrow I am looking after a friend's grandchild for a couple of hours mid-day, so I would message and say something on the lines of 'What have you in mind for tomorrow, because I'm tied up between 12 and 2.'
If you are talking about tomorrow evening, then much the same applies.-, e.g 'I'm busy until 7 etc..... so what have you in mind?'
Best I can do. Keep it light, but be grounded.