I’ve been down this road many times, with peers losing children either to suicide or accidents. The best thing is to just say how sorry you are, give them a hug. If the son was a friend of your children and spent time at your house, round up any memento you have—photographs, artwork, etc., and give them to the parent. If you write a condolence letter, recount any memory you have of the child from the past. Most parents don’t want people to act as if their child never was.
25–30 years later, I still write notes to the parents of high school friends who died in their early 20’s and tell them I was thinking of their child, and miss them still.