I'm insisting my fiance's dad to pay for his tuition
Did your fiance and his father have an agreement that he would pay for the tuition?
I told my fiance that his dad can change his life.
It's good to encourage him to speak to his father and see if he can help pay for his tuition because it could save him a lot of money on costs. If his father can't help, then your fiance will need to find the money another way.
Whatever, it's my decision and I think the father should pay for his tuition.
It's great if he can pay for it, put his father may not have the financial means to help. It's good to ask, but you have to accept that his father may not be able to afford the cost and the two of you will need to make a different plan to pay for the tuition.