Grandparents have the right to ask leave of the family court for them to make an application for contact (S.8 of the Children Act 1989) I'm not sure about the step son issue. Would it not make more sense for your H to make the application, as presumably the baby's father is his son.
If the court agreed to your application, the situation would be assessed by a CAFCASS (Children and family court Assessment Service) social worker, who would make recommendations to the court.
However as Lottie says, even if contact is awarded to you by the court, the parents can still be very difficult over actually allowing you to have contact with the child. Disobeying a court order is of course contempt of court, but courts are very reluctant to give a custodial sentence to the parents because of the effect on the child/ren.
If you are thinking of going down this route, you will need a lawyer who is experienced in family law. I don't know what your financial circumstances are but if you qualify for legal aid, I would do it sooner rather than later because the govt are withdrawing legal aid for private law cases.
Disputes in the family courts are always difficult and I really think that if at all possible, you should try to negotiate with the parents, rather that going down the legal route, but of course what you decide to do is up to you.
What age is the child btw and did you have a loving relationship with him before this conflict. If he is old enough, his wishes and feelings should be taken into account.