i think bringing his friends along would help.
but honestly, don't force it. if he's not into it, he's not into it. if you force something on him, he will just resent it.
my dad started me younger, 8 or 9. 13 is a tough age, i know myself i was becoming mroe interested in girls, music etc. at the time hanging out with dad just didn't seem "cool" at the time.