Jazz won't go to a doctor because he's both a kid and a teen.
Jazz isn't a teenager, he's in his 20s now. Unless you were talking about his maturity level, then yes, in essence, he is still a kid/teenager.
Could also be he's emotionally scarred from his botched SRS surgery. The pain he had (and still could be in), probably the shock of seeing what they did, the fact almost immediately after the surgery the whole thing split apart, the multiple revisions that were undoubtedly equally painful each time. And the fact he's basically forced by his fucked up mother to never speak the truth about his surgery and all the complications, all this could easily culminate in why he's scared of doctors.
I also wonder if Jazz got fatter, so that he would be unable to dilate. Remember he hated dilation, and his psycho of a mother, on camera, told him harshly if he wouldn't do it, she'd shove it in there herself. And Jazz is not the only troon who hates dilation. Go on the surgery subreddits, and there are so many posts of TIMs complaining about how painful dilation is and how much they hate it. And of course it's going to be painful. You're shoving a plastic dildo into a wound to keep it from healing.
That was a big push for minors to get SRS. So parents could make sure their kid sticks with the dilation schedule. Which of course, unless you're a psycho like Jeannette, most parents hopefully aren't the type where they'd do it themselves if the kid refused to do it.
And I do wonder if Jazz really did experience sexual abuse, because a lot of kids who are victims of CSA use eating disorders to cope with what happened to them. Some starve themselves, but more often than not, they binge and get fat, in hopes of being undesirable.