New partner, great guy and friends for years.
Now we’re in a relationship and he’s opening up to me and he’s revealed;
Serious sexual fetish.
Depends on what it is. Is he forcing you to participate, or wheedling you until it's hard to say no? That's a red flag.
Mental ill health requiring medication.
There are many different types of mental ill health, and many degrees of severity. How well controlled is it, does he cooperate with his medical team? You'll be extremely lucky if you yourself go through life without having mental ill health at some point. I have mental health problems, medicated, and I'm a fully functioning member of society, my family, and my (stressful, fairly senior) profession.
Past relationships with men.
He's to some degree bisexual. Why are you being biphobic? Why is this an issue? I suspect you may have some false beliefs about bi people.
And have you thought that the mental health issues could result from a childhood of suppressing his attraction to men?
Offending behaviour in his childhood.
Like what? I know some people who did minor shop-lifting, or drugs, and they wouldn't dream of doing it now.
Some people grow up in families where offending is almost inevitable. As long as he's turned his life around, why should it matter?
He’s a great guy, but AIBU to think that there are too many red flags to take this relationship forward?
I don't see these as being red flags. If the mental health issue is very severe it can put pressure on a relationship, so it depends on degree and control. The childhood offending again varies so much in degree that it's impossible to say - if he murdered someone, tortured someone, or raped, then I'd be thinking carefully. If he stole magazines from WHSmith I would just think he was a bit of an idiot as a kid, or had a troubled upbringing.