You can use the skull candle to get into his head. The rub is getting a "genuine" apology. Using rootwork to compel him to apologize may not bring a true response. Likewise, Clarity may only be effective if he is capable of seeing his shortcoming. I would try Clarity first, and call on the work to show him the hurt he has caused, the injustice he has perpetrated. If he is so apathetic that he is unable to acknowledge his wrongdoing, I'd break out the hyssop and batten down the hatches for some rough work.
Curiously, what is coming to mind is a spell available on the Lucky Mojo site for getting back money owed. This apology is a debt owed, so you may well be able to call on the same spell. Of course, instead of calling for money to be repaid, you will be calling for him to bring you the apology that he owes you. Check out the spell here:http://www.luckymojo.com/payme.html
Maybe this will help.