Short version:
Do you want to have a relationship with your son? If so, you try.
King clearly matters to your son, so if you care about your son you should try to put your own shit aside.
This isn't your son trying to ruin your day, and that text message makes it abundantly clear this isn't just some casual request, or even the first time he's asked you to try and give King a chance. If you can't see that this is important to your son, you have a ton of self relfection to do.
Longer:
Is the age difference really your only reason to dislike him?
Disclosure: My wife is around 15 years older than me. We've been married a few years short of a decade.
I won't pretend the age gap between your son and his boyfriend isn't concerning (or even that the one between my wife and I isn't). It is.
They are both coming from entirely different place in life. They both have vastly different life experiences. King having more life experience and likely already being well established creates a power imbalance that could be dangerous or used in an abusive manner towards your son.
All of that said, do you have any reason to believe any of that is actually occurring?
If not, then treat it for what it is: a normal relationship with some weird attached. Not a big deal. Every relationship will have its quirks, your son's just so happens to be a publicly visible one.
At this point, your kid's an adult. You can talk to them about your concerns regarding the relationship (once), but beyond that, you need to let them make their own decisions. Whether you think they take it seriously or not, whether they reach the same conclusions or not, it is their own decision to make. Their own mistakes to make.
Your son has already made it clear that he doesn't need your approval to have this relationship. At this point the only thing your grumbling/grumpiness will do is drive a wedge between the two of you.
Another disclosure: My personal story of "that one big bad crazy ex" includes my parents and numerous other people trying to warn me. I didn't listen. I still lived with my parents at the time, and what their constant grumbling etc did was drive a massive chasm between us. I moved across the country to get away from them, effectively crashed on a couch with no safety net whatsoever, and almost ended up trapped with the crazy when shit hit the fan. There was literally nothing they could have done or said that would have made me see things their way. I had to reach my terrible conclusions myself. There is still lingering baggage in my relationship with my parents nearly 15 years later.
The important thing is that you be there for your son in a way that doesn't drive him away. But for his and your sake, really deeply reflect on what your issues with his boyfriend are.
Dangerous is dangerous. Shitty personality is shitty personality. Age gap on it's own is just weird as shit, but weird isn't inherently dangerous.
A classic