My condolences. Your grandson’s DD-214 entitles him to a funeral with Honors, and burial at a Veteran’s Cemetery depending on the nature of the discharge, and available room. For his son, I would suggest going to ssa.gov. That would be the most comprehensive site, and should point you in the right direction.
I have been to a few veteran’s funerals over the years and the honors have been provided by the VFW or American Legion.
The only one that actually had a Marine Honor Guard was for a Marine killed in combat.