Because you love your son, you don't want him to go through another unhappy marriage and you know this could easily happen if he marries someone he doesn't really know. If you don't say anything and the marriage does turn out badly, you'll always wonder if you could have spared him the pain.
At the same time, you need to be wise in the way you approach your son. If you attack him and challenge his judgment, for example, you'll probably only get him upset and he won't listen to what you're trying to tell him. Instead, let him know that you love him and respect him, and tell him you hope he will think through what he's doing because you know this is a major decision for him.
In other words, be as positive as possible. Tell him you are pleased he is thinking of remarrying (if you can honestly say so) and you want him to be happy if he does. Then share your concern, gently but clearly. The Bible says, "A gentle answer turns away wrath, but a harsh word stirs up anger" (Proverbs 15:1).
The most important thing you can do, however, is to pray for your son, and urge him to pray about this decision as well. If he has never committed his life to Christ, encourage him to do so, for he needs the solid foundation only Christ can give. And if he does remarry, welcome your new daughter-in-law into the family and pray that they may find happiness together in Christ.