It doesn't sound like you ex has any fortune. If his wealth is basically in the home that he shares with his current wife, then leaving that to her sounds pretty normal to me. This second marriage is only 3 years old, and it may fail. Statistically, second marriages have less chance of survival than first marriages. If your ex is reasonable healthy and probably likely to live for another 20 years or more, then a lot can happen between now and when he passes away. The nature of his relationship with his son can change too. I would stay completely out of it, if I were you. This is between dad and son.

If your son hasn't been going around worrying about his dad's will, I wouldn't go instigating this as an issue. The less parents do for their children, the less the kids feel obligated to worry about the parents. (Not always 100% true, but often is.) Right now, your son hasn't quite reached the age of worrying about things like his father's will. By the time he is 30, he will have it all figured out. I'ld say that it's best to leave him to awaken to the money realities of family ties in his own time, in his own way.

Bringing this up as an issue is likely to just stir up resentment in your son toward wife #2, or hadn't you thought of that? For him to feel resentful will just poison his relationship with his step-mom, and that will just further alienate him from his father. I would just drop the whole subject, if I were you. It would be different, if your son was a special needs child. But, if he's healthy, your ex has no compelling reason to make "provision" for him. If your son ever initiates discussion about inheritances, you can say, "Well, you're not real likely to inherit anything from either your dad or me. So, if you want to have something in this life, you better plan on getting it for yourself."

The best thing your ex can do for your son is to manage his assets well enough, so that, when he becomes old and sick, he has the means to get the help he needs and not become a burden to his only child.