Hiya,
A shoebox or something similar will do. You don't need a special box for it. Needs to weigh no more than 2kgs - they used to give a bit of leeway but they really don't now so weigh it before you go to the Post Office if you can.
Remember to only use the BFPO number and do not put Afghanistan on the parcel. Make sure you put a return address on the underside or side of the box otherwise it will not be sent. Try and pack it out as much as you can cos it will get bumped and bounced en route.
As for contents. Haribo sweeties always go down well! Biscuits, crisps, magazines, Super Noodles! - just munchies really.
He'll appreciate it and whatever he doesn't like, somebody else will
Also, if you're letter writing, you can send E-blueys (sorry if I'm teaching you to suck eggs there).
EBluey
You write it out like you would an email and it's printed out on site so will usually get to them within 24 hours, sometimes within a few hours.
On that site there should be a 'do's and don't's' where parcels are concerned as obviously some things can't be sent.
Hope that helps - safe return to your pal

Waiting for my rocket to come.