My daughter died and I am trying to get custody of my grandson (7years old) I helped raise him with her because his biological dad abandoned them when he was born. He now wants custody over my grandson who has never seen him... Do I have a chance to win this if we go to court?

Probably yes, probably no.
According to American laws and Court practice , there is "parental right doctrine," or the "parental preference rule".
Nonetheless, there are many court cases where the judges have granted custody of children to third parties, such as grandparents, close relatives, or other persons with whom the child has been living for an extended period of time. .
The custody of the grandchild automatically goes to non-custodial parent unless he or she has been found to be unfit. Even in states that allow grandparents to seek custody when a child is not in the custody of a parent, the mere presence of the child with the grandparents at the time of the custodial parent's death does not add to the grandparents' rights.
Some states find that the best interests of the child may require placement with a third person even though parents have the primary right to custody.

You have a chance, I think a pretty good chance but I would strongly advise consulting an attorney.

Consult an attorney. There are many attorney who will give you a free 30 minute consultation which is very helpful. I think that you have a good argument for custody and there could be a chance. However, your best bet would be to talk to an attorney for legal advise on this matter.

Attorneys in my area no longer offer free consultations. Blood relatives come first when there is a custody dispute - as GV said. Where is the child now?

The Courts will make a decision based on the best interest of the child - that might be "split" custody, supervised visitations with the father, anything in between.