I think you can prove paternity by supplying a sample from his body for DNA testing. Perhaps he left a hair brush at your house or clothes that he'd worn.
You can also provide paternity by having his parents provide a sample for DNA testing along with yours and your son's.
I don't know the rules that SS require or if DNA testing will be enough of a proof but you could ask.
I don't know if an attorney could help. I think I'd first try being very open and direct with his parents asking them why they haven't filled out the paper work. Do they have the papers? If not could you get them and give them to them and wait until they've filled them out and then mail they yourself.
Having their son die is no doubt painful. Perhaps filling out papers related to his death is difficult for them to do. Assume that they want to help their grandson get the benefits to which he's entitled. Don't make threats. Threats rarely work. They make people angry and less cooperative.
And please don't feel that they are denying your son. If they're wanting to visit with him they're not denying him. I think that there are other possibilities for them not filling them out. One that immediately came to my mind is that they're procrastinators. I nearly always have difficulty completing government forms. I've been late with my taxes more than once. The death of their son is much more emotional than filing taxes. Give them the benefit of the doubt. There is no deadline for filing for death benefits.
If you can't get them to fill out the papers by being calm and persistent then call the SS office to find out what the next step would be. I've had frequent contact with the SS office and found them to be helpful.
Someone suggested that they might want the benefits for themselves. That is a legal impossibility. The benefit is for the child of the one who is deceased.