some guys can only understand if you spell out black and white.
write down your expenses, needs, and future savings for your son and family on paper. then deduct from the total income and see how much is left. exaggerate your expenses and saving needs a little so that the money that your hubby earns seems to be tight if he gives that much amount to his parents.
however, i feel if all the expenses and savings are met, then any extra money, you should give him the liberty to spend it in any way he wants, including giving them to his parents. (i dont't know how 'above average' your earnings are).
on the other hand, if such expenses and savings are not met comfortably, then you should have a good talk and write an accounting budget statement so it will look scientific and more convincing.
i see nothing wrong in giving money to parents, big or small (of course must be within our means). the debt we owe to our parents for bringing us up cannot be repaid, whether they take care of their grandchildren or not. (unless of course, parents take money to gamble or drink alcohol, then giving money or not is a different story)