Phone him up and do a bad G man accent, do this every day until he changes his number.
Wait 2 months, phone him up at the new number, and say the following-
Well well well... isn't this just like.... old times...
Alternatively, get a job with him. Wear a white wig, chase him down corridors telling him that they're waiting for him... in the tesssssssst chammmmberrrrrr