Hi MBGal.
I have a few thoughts that could be constructive.
Ultimately abortion is your decision. However, it's his responsibility as much as yours and I think he deserves to at least give you his opinion on the matter, whether you care what he thinks or not.
This is something that has happened to you both, he just doesn't know it yet.
I think the fact that you are "torn" means you feel this even if you don't understand it.
One more reason to tell is that no matter how easy morally or physically it is for you to do this, there is bound to be some pain and guilt involved, and you shouldn't have to bear that alone when you are only %50 responsible for the situation.
You made this happen together, you should deal with it together, for the sake of all concerned.
Being a man, many women will dismiss my input as being irrelevant, but I can assure you that if I had been in such a situation, I would have supported the girl whatever her decision - it is her body and her future most directly effected, but if I found out later that I had been left out of the matter entirely, I would be absolutely furious.
Yet one more reason is that guilt is the most destructive of all emotions and you have to live the rest of your life knowing that you did not tell him. The short term pain of having to tell him now is much easier to bear long term.
So, in summary I feel that the decision to have an abortion is yours, but that he has a right to know about it and to take responsibility for it.
You have said you may tell him. I hope that you do.
I hope I have been some help.