You know your friend better than we do. Do what you think is right, and make sure you’re open to accepting that you’ll get it wrong sometimes, and then self correct.

I don’t think there are any phrases that will seem offensive. People use “don’t break your balls” when speaking to cis women or “getting on your/my tits” (🇬🇧 expression for annoyance) when talking to cis men, for instance.

Depends on their personality. My co worker is trans and she DGAF so I purposefully say stuff like that for the memes. But that’s just how we talk.

You can always ask.

Well the answer is a deceptively simple “it all depends on the person”. The line for when it crosses over into dysphoria isn’t a universal one. Really the only way you would know if you upset him is if he told you. So if he hasn’t ever said anything to this effect, don’t overthink it. It’s a common idiom.

The good rule of thumb is “would I be making a genitals comment to a cis person in this same situation?”. Lots of guys will make dick jokes, but how often do those same guys ask each other the medical history of their junk? Not often I’d imagine, and the times that would happen would be for a specific reason.

Might be safe to err on not making references to the genitals they were born with, but everyone different. As a trans woman I will happily use my vestigial teeny tiny penis as a punchline if I know I am among friends. I’d just never presume to make that joke about another trans woman.