Minimus....This is a hard one, I know, but since your friend is looking to you for some support then the best thing you can do is just be therefor him....as that friend. Let him talk...listen to him from your heart....call often and check on how he's doing, give him opportunites to unload all the stuff he will need to get out...regrets, guilt maybe over some things, anger at the whole thing...he will experience many emotions and they will all need some form of expression. Mostly he needs to talk it out, and you can be that sounding board for him. Be patient with him, grief takes a long time to go through, and I imagine especially so when its a child...it seems very unnatural for our children to die before we do!
He will also need to mourn...grief and mourning are not the same thing. Grief is a term for the thoughts and feelings that are experienced with oneself upon the death of someone loved....it's the internal meaning given to the experience of bereavement. Mourning is taking the internal experience of grief and expressing it outside of oneself....its the grief gone public or sharing one's grief outside of oneself. Both are necessary in order to work through one's grief so that it doesn't become baggage one carry's around the rest of one's life. You can help your friend find ways to express his grief...give him that safe place to go to...go with him.
In our culture men have a harder time in expressing their feelings on most things, and grief is no different.... He may be looking for some sort of spiritual help from you which you can't give honestly, but you can help him to give meaning to the life his son lived even if you don't have answers for the future life expectancy as in a resurrection or any of that stuff.
I'd suggest grief counseling, but as a JW he probably won't go....they are they're own support group for that. I am currently going to counseling for the death of a parent 17 yrs ago because I wasn't allowed to grieve or mourn properly at the time of their death, largely due to the fact that since my parent wasn't a JW then no one felt I should be grieving since they would be resurrected and I'd see them in the new system.... I hope your friend doesn't fall into that trap! I have lots of material I can send you to help your friend with if you're interested...PM me if I can help with that. Also, if you have a local hospice then they also provide reading materials for helping people through their bereavement process.
Mainly just be there for him.....
Bythesea