My personal experience with being on T is that I have changed in subtle ways, but not in major core ways. My core values and personality are the same, but some of my perceptions and reactions have changed.
I'm more assertive, but less angry. I'm much more relaxed and easy-going. I don't cry nearly as easily as I once did. (Thank goodness!) I'm more out-going and less anxious. That's not to say that I've suddenly become a social butterfly, but I'm more likely to join in on conversations, etc. I don't get angry as easily, and I don't stay angry as long, but I have to watch myself when I
am angry, because I'm more likely to say something unwise.
My perception about sex has changed in subtle ways. That one is hard to explain without being misunderstood. It's as if there is more of a separation between love and sex than there used to be. Don't get me wrong. Sex can still be a wonderful, intimate, almost sacred thing with the right person. And if I were in a relationship I would never betray my partner's trust by cheating. But at the same time, it's as if there is no longer an absolute imperative for love and sex to come in one package. Given the right situation, I might entertain the idea of "friends with benefits", whereas that idea was totally alien to me pre-T.
I suspect, though, that people who are close to me may perceive more of a change than I do. After all, I spent a lot of years hiding much of myself and trying to play a role that they would find acceptable. I'm through with that.
And yet, interestingly enough, now that I am being perceived as male more and more often, I find myself becoming more willing to express my feminine side. I actually contemplated buying a pink shirt, the other day, for the first time since I was about 8 years old.

And if people have problems with me being non-competitive, cooing over cute baby animals, or expressing my concern for someone's welfare, well they can just shove it. I'm not going to hide inside myself anymore.
What was I just saying about being more mellow?

Sorry, rant over. Yeah, it may be a bumpy ride, but in the end the relationship will likely be healthier for it. Better to go through it now than to be faced with 10 years of built-up resentment when it all comes out further down the road. And that is what will happen if he doesn't deal with this now. Unfortunately, it doesn't just go away. Take it from someone who put off transition for 30-odd years.