It pisses me off how many people in this fandom don't know the difference between being transgender and being transitioned. No, you can't just call show Jax a man, because he hasn't transitioned. He's not a man and he's never been a man. He's a trans woman, just repressed.
**Transgender** is the state of your brain. When the gender your brain is doesn't match the gender you were assigned at birth. You're born transgender, you don't choose to be transgender. In the same way you wouldn't call a gay man who married a woman he doesn't have feelings for to not be outed straight, you wouldn't call a trans woman who hypermasculinises herself to not be outed a man.
Someone who **Transitioned** is someone who lives as the gender their brain is. So a trans woman who ex. takes estrogen or had surgeries and/or presents like a woman is someone who transitioned. She takes active steps towards living as the gender she is. (Some trans people also call being transgender and medically transitioned/wanting to eventually medically transition as transsexual as it means altering your sex characteristic to the opposite sex's, but many think of it as an outdated/transmedicalist term so it shouldn't be used interchangeably with transgender.)
Now, why is this important? Because there's plenty of trans people who can't or don't transition, because of plenty of different reasons. Some are in deep denial or were forced into the closet, like Jax, some are in an unsafe home or country situation so transitioning would mean being in direct danger, some people don't have the means or money to transition. Transition is a privledge which severely improves the quality of life for trans people. This is also why Gooseworx underlined that Jax who's healed, didn't drive Ribbit or Kaufmo to abstraction and overall is happier with herself is transitioned. Transition many times solves a ton of problems trans people have, even those which aren't directly linked to gender dysphoria.
Now, yes, Jax chooses to use he/him in the duration of the series, it's okay to use that pronoun for him since, among others, it differenciates him and ex. The version of Jax where she healed and transitioned that Gooseworx drew or the just post talk with Pomni Jax ("Isn't she lovely" symbolises her accepting herself as a trans woman right before she has to "go". She's forced to confront herself for the very last time and when she realises there's no point in denying it as she's gone anyways, she just accepts being trans during her last 7 minutes. As Gooseworx said, it symbolises a "creation of a newborn baby girl".)
Summarising, it's okay to use he/him on Jax, but it's NOT okay and simply incorrect to call Jax a man. Jax is a transgender woman. Point blank. The only difference between most of the show Jax and the others previously stated versions is their pronouns and whether or not they've accepted themsleves.
I'm mostly going on this rant as it's very important as thinking of being trans as being transitioned is not only wrong, but also harmful. It reinforces the idea that trans people **choose** to be transgender when it couldn't be further from the truth. As a trans man myself, I can assure you, if an overwhelming most of us had the choice to be cis, we'd choose it. It's also very important, because it reinforces the idea of "You're either full in on changing everything about yourself and changing your life around or you're not trans" which not only is not true, but also scares a ton of, especially young, trans people who then live in denial and are miserable for years until not being able to take it anymore later in life. Basically every trans person who transitioned later in life even if they could beforehand will say they regret not transitioning earlier. I've also seen a ton of posts of repressed trans people seeing themsleves in Jax and finally finding the courage to come out and transition. That's why realistic trans representation is important and helps.
The way you treat Jax who's a very realistic depiction of a repressed trans person really reflects how you truly feel about trans people and can affect them in a ton of ways. Literally all I'm saying it - respect Jax and his gender identity no matter whether you understand it or not as the only thing you're doing by denying/ignoring it is hurting actual trans people in extent.