The father of the child is LEGALLY obligated to pay the amount of money stated within the court order - that is it. Should he, however, choose to contribute more to the support/care of his child/children then God bless him. Out of curiousity, why are you so interested in what amount he is choosing to pay towards the care/support of his children - I understand that you are engaged, but the financial choices he makes regarding his children are his own.....is he looking to reduce the amount he is paying or do you feel he is paying too much? If it is a matter of him wanting to reduce his financial output to the children then I would suggest he simply return to paying only the amount ordered and leave it to the ex to petition the courts for a modification to increase the support to include the child's preschool costs (which by the way if it can be proven he can afford it -as he obviously can if he has already been paying it - there is a good chance it would be granted on the grounds that the child's increased age and therefor schooling requirements would qualify as a change of circumstance.)
Bottom line is this -- legally he is under no obligation to pay more than ordered -- morally he can choose to contribute as much over that amount as he sees fit.
Good luck and God Bless