I think you might want to sit down and jot down your goals and objectives for this kid before doing anything.
If the end-game is to support his higher educational aspirations with an eye towards economic independence, that’s a goal.
If the end-game is to foster intellectual inquiry and help him develop his artistic or scientific skills, that’s a goal.
If the end-game is to follow the letter of the law in terms of your support- as specified in your divorce agreement, that’s a goal.
If the end-game is to ensure parity among all the children- bio and step- in terms of financial support, that’s a goal.
If the end-game is to use the college years as a way to rebuild your relationship as your kid becomes more independent and less influenced by your ex wife, that’s a goal.
I’m detecting that you are looking for a bunch of things simultaneously- to be treated both as bio dad and active participant in your kids lives; to ensure parity among all the kids; to help each kid become educated and independent; to somehow punish your ex-wife; to get to have veto power on the kind of/type of education; NOT to have to pay for something you didn’t get to choose or approve, etc.
And I’m here to tell you that you aren’t going to be able to achieve everything simultaneously. So figure out your end game before proceeding.