I have been growing this guy for 6 years. He's gotten a little too top heavy and today has fallen over and the top piece (about 9") broke off. The break was very even.
I have no idea where to begin here please help. Can it be grafted? Is it doomed?
You can graft it back.
If the wound has already dried, use a very sharp knife, as in a proper sharpened and honed chef's knife, and slice off a pickle slice off the top piece and bottom piece.
Wrap the cactus gently but firmly with cotton string, your common classroom white string is perfect for this.
Wind the string around the cactus avoiding the spines, and criss-crossing the wound several times covering an inch above and below the wound making the string both the bandage as well as the support holding it in place.
Leave the string there until it looks like the cactus is going to start growing around the string, then it is time to remove it so it doesn't scar the cactus too much.
It's a pity we can't see the pot properly - seems likely that, as it got heavier at the top, it might have needed a bigger or wider pot to prevent its falling over. Replacing broken parts of plants can be done, if done immediately after the break occurs, but the join somtimes needs 'splinting' and definitely needs taping; as this plant is prickly, it doesn't seem likely this method would be possible without causing more damage to the spines.
Plants get broken all the time in their natural habitat and don't necessarily die, so all you can do is wait and see if it heals itself. It might, though, need a bigger pot anyway... and it's worth following Anubis' suggestion with the broken part.