Depends on the situation. Sometimes the ex-spouse (lets say you) out of habit, allows the ex too much control and the new fianc (or spouse) can see it better than you that some strings are still attached. On the other hand, the new fianc may be overstepping their bounds by asserting their role when they haven't yet established their place in the forum of that long term (sometimes contentious) relationship. Maintain rapport by really listening to your fianc. As your new potential life long partner you should value their opinion whether or not you agree with it. If that same respect is not reciprocated to you then your fianc is not being reasonable. Being that your ex and your child are not going away, you must resolve this conflict before you get married. You (not your fianc) will always be the one responsible for how
I would explain to him that your son comes first in such matters and that he (the fiance) is going to have to learn to live with it. If he's jealous of your ex, and jealous in other ways, I'd give second thoughts to your engagement.