@maone - It must be so reassuring to know that your son is alive. Although I know that you want to protect your daughter-in-law and grandchild and this is commendable. But what if he returns and you haven't told them that you know that he has been alive all this time.
I can only try to put myself in your "intellectual" place and think what I would do. I am in no way trying to judge you. I am trying to make sure of all possibilities. It might also help them move on more peacefully, as it hopefully will do for you. And their is also the legal side. What if your daughter-in-law wants to remarry? She would have to have him legally declared deceased to go forward and would be protected if he suddenly turned up. He wouldn't be able to take anything from her either.