Although it's generally taken as the norm that fully comp insurance offers you third party on other vehicles with the owner's permission, as the others said they've started quietly excluding it from policies now (in my experience generally for the under-25s).
It'll be explicitly stated on your insurance documents if you can, and in some cases it may explicitly state that you can't, but it might not say anything at all. If in doubt after looking at the documents phone them and ask.
If you're not insured some insurers will add a named driver for 1 day, or 3 days, or whatever... so that may be an option if he speaks to his insurers.