Is it possible for you to have your grand son, some weekends on his own, so he has a bit more time on his own with you, and without the others? You say you have seen nothing yourself but are aware it goes on........how are you aware?
If you daughter has made previous bad choices in men (she must have had the ones I didn't pick) how did the boy cope then? Maybe he feels insecure, and needs help to cope all round and not just with this issue.
Blended families are the norm now but that doesn't mean they come together seamlessly, they can be really hard work and I guess the other kids involved have their own views and issues too.
It might help if you could have your grandson, so the other boys don't feel they are competing with him for their dad's time at the weekends (I am assuming they live with their mother) if your grandson feels miserable, he may be extra sensitive and subjected to a different style of parenting.........the permutations are endless.
All very well for modern relationships to be about adult happiness but so often the poor kids are trapped in the middle and just waiting for the repeat performance of what happened last time.
I wish you luck with this one, it is so hard to help them.