I'm going to have an abortion without telling my boyfriend about it.
Why wouldn't you tell your boyfriend about your abortion?
I don't think he would approve and I'm afraid of his reaction
While it's your body and ultimately your choice on whether you want to carry your pregnancy to term, it isn't appropriate to hide that information from your boyfriend. As the father of the baby he does at least deserve to know and to have a chance at making his preferences regarding the pregnancy known. If you are afraid of his reaction and he has a prior history of abuse, then it would be best to look to ways to leave the relationship since it isn't healthy to remain with an abusive partner.
I think it's my choice and I don't want him to make a big deal about it.
If you are in a relationship then it's only right that he gets a say in this as well. It's his child as well as yours.