It depends really, how co-operative you are feeling.
For the record, it costs around �30 to have the PO re-direct your mail for six months - I know, we are moving next week.
If you want to be helpful, write to him and say that from today, you will keep all mail for one week, after which you wil destroy it. This will exclude all obvious 'junk' mail such as catalogues and circumars, which will be ddestroyed on receipt. Say he may collect post once a fortnight at a time and on a day designated by you - no exceptions. This agreement will last for one month only, after which all mail will be dstroyed on receipt.
That should do the trick.