This is always a tough situation, and I have seen it many times. What I would say is that he deserves the right to know. While no one can ultimately tell you to get or not get an abortion, I think he should have a say in the matter. I just think that even if you do not want this child, shouldn't he be given the right to choose whether he does? He may want to raise the child on his own, and I think he deserves the opportunity. I would say it this way...
If you want to keep the child, tell him and let him be a part of the child's life even if you are not with him in the future. A child needs a father, and from a medical standpoint, might need information from him down the road.
If you do not want to keep the child, tell him and let him decide if he wants to. If not then consider adoption rather than abortion please. There are many "mothers" out there who can not have their own children, but desperately want to be a mother. Give that child a life.
Tell the father, he deserves to know.