] Yes, his logic *is* rather weak. I think what he means is that

] b) Mr. Bennet expected that his son would help him break the entail (not a foregone conclusion, IMO), thereby freeing up more money to leave to Mrs. Bennet and the girls.

I agree with you... I don't think there was any legal way in which the son could have been forced to help break the entail.

And, as Hilke has already observed, even with the entail broken, there wouldn't be any more money in the end. Mr. Bennet would have been far better off to save regularly for the future support of his family.