Gender fluid actually means he is transgender. You are confusing the terms (and I don't blame you)... if he feels he is more gender fluid, he's probably not transsexual. But gender fluid definitely falls under the transgender umbrella. I had typical transsexual feelings and struggles from basically when I remember, so I can't help you much. I do know a few gender fluid individuals, and they're all strongly against any body changes. All I know said they never felt any body dysphoria, unlike transsexual people, who are defined by having severe body dysphoria, and urge to change the body apperance with surgery and hormones. What you two need to do is find an experienced gender psychiatrist who will help you answer your questions, and clear the situation.
With people transitioning at older age it is mostly a case of "type 2" transsexuals, who are in deep denial for very long time, like 30, 40 years or more. Most likely they were married, have children, are (primarily) attracted to the opposite sex of their biological sex, do everything they can to suppress their feelings and keep them from anybody knowing, and in most cases do their best to present on outside as typical hetero-normative masculine men. There is a lot more to discuss and look into between you two, and really the most intelligent approach would be talking to someone with professional background. If you find out he (she in this case) is really a transsexual, a transition is the only option. The sooner the better...