If your son was married to his former partner at the time of their child's birth he has 'parental resposibility' automatically.
If the couple weren't married but your son's name appears on the boy's birth certificate he also has parental responsibility automatically.
If they weren't married and your son's name doesn't appear on your grandson's birth certificate then (assuming that his former partner won't voluntarily give him parental responsibility) he can apply to a court for parental responsibility:
https://www.gov.uk/parental-rights-responsibilities/apply-for-parental-responsibility
Once your son has parental responsibility he still won't have an
automatic right to contact with his son (although his former partner would still need to keep him updated about your grandson's well-being and progress) but it would be an important factor if a court was asked to make a contact order.
However a court won't consider such an application unless your son can show that formal mediation has been tried first, so that's the route he initially needs to take. If that fails (or his former partner simply refuses to have anything to do with it) he can then seek a court order:
https://www.gov.uk/looking-after-children-divorce