This is hard on a parent for sure, we expect our children to outlive us. Prayers will be going your way and theirs.
What you can do though is just be there to let them talk about their son or, just to cry.
If something happens while you are together and it reminds you of your nephew in a good way, don’t be afraid to say, “I remember how John(don’t know his name so…) would make us all laugh by making faces at a child who was having a hard time waiting at the Restaurant” or whatever… don’t be afraid, memories at this point are all they have and shared ones are always good (of course, they have the knowledge that they will one day see their son in Heaven but that is not always a comfort to all - it was to me and to my husband - but not all people are or they have heard it so many times it is getting old, they want their son in the here and now).
If I may ask, what caused him to pass away in his sleep? Was he ill, did he have a disorder that would cause them to know that he might die young? If so, they may be happy to have had him for the 24 years they had him too. Or, maybe they need to be reminded gently of this fact. If it was something totally unexpected don’t go there.
Again, just let them talk about their son, you talk about what you remember and above all, let them cry if they do. Make sure you have a box of tissues handy at all times.
Brenda V.