***** NOT A TEACHER *****
(1) As I type this, no one else has answered. So may I try?
(2) I agree with you that if you are SURE that you will see him on the weekend and that this is a REGULAR occurence, then perhaps "Have a nice weekend" is not that appropriate.
(3) How about:
(i) Have a nice evening.
(ii) See you soon.
(iii) Enjoy yourself.
(iv) Take care.
(v) Don't do anything I wouldn't do. (a joke between close friends)