How do you apologize? Saying sorry and doing it again is really not an apology. Part of a proper apology is promising that it won't happen again. If this situation keeps happening than your apology is not worth much and that is what he sees.
He is wrong to use bad language with you but you are also wrong in commiting to things that you may not be able to do.
I think you should give him a proper apology not for the incident but for the situation and your failure to meet commitments that you made in the past and promise that the situation won't arise again... in other words that you will not commit to doing an "errand" if you are not sure that you will be able to do it.
I found a great site on perfect apologies when I messed up (with my SO) and read about how to apologize effectively and it was really helpful, and best of all it worked.
Here are the basics:
A proper apology always:
gives a detailed account of the offense
acknowledges the hurt
takes responsibility for, and ownership of, the mistake
offers an explanation that recognizes your role in the situation
includes a statement of regret, shows humility or remorse
asks for forgiveness
expresses a commitment to change and a promise that it won't happen again
provides restitution, or token gesture in line with the damage
