Ah! I have a lot to say on this topic.
For background: OP, I just recently quit my 6-figure job to pursue my true professional dream. I'm essentially starting my own thing from the ground up in the industry that I love (I'm moving from computer software to adventure sports.)
I love every single day that I wake up. Every one. I roll out of bed directly onto my desk and continue on working for the next 10-12 hours. Unlike my old day job, I don't even notice the time go by as I'm so goddamn happy pursuing what I feel is my life's true calling.
Some changes in my life: I no longer have an income. Have only a few months of savings left. Receiving food stamps and welfare. But I am SO. DAMN. happy.
This is what I figured: if my business fails, here's what I lose: what? a year's worth of salary. I am 29!!!!! If I'm making 120-150K now, I can easily make that again in 6 or 12 months from now if I get a new job. No problem. I have the rest of my life to make money.
Here's what I don't have the rest of my life of: optimal time to take risks. I am 29 and female. And in a few years, I may want a family. And children. And then it will be VERY irresponsible to quit a good job to pursue a dream when there are tiny little mouths to feed and to take care of.
That's if I lose. If I lose I lose 1 year's worth of salary.
But if I win?
We don't know if there's an afterlife OP. Or at least I don't. If I don't make my dreams happen now- or at the very least TRY TO- then when the hell do I get to give them a go? That's where your boyfriend is at. He has a dream that burns inside him. In my incredibly biased view, he should absolutely pursue it. He is correct that if he does not, he will hate himself for it.
His Masters in a marketable field will still be there in 1 or 2 or 3 years from now. His masters will never leave. You say you are afraid he will never get another good job again but that's unfounded: he can always take more classes, or even go back to school if necessary. That skillset is rare and he is young, he can always go back if he wanted to.
I think what you're *truly* afraid of is your change in lifestyle and you should be. In my case, I am very single. I was going to buy 8 apartments this year. Now I'm on food stamps. My plans changed. But I am alone. I respond to no one. In your boyfriend's case, he lives with you, and suddenly your own financial security and stability is being turned upside down. You are very correct in feeling scared and hesitant.
That being said, in your 20s and with no debt nor kids, I (and I may be super downvoted here) do not believe you have the right to tell him to not follow his heart. You have the right to talk to him and explain why this is so scary for you and discuss all the possible outcomes. But outside of this, there really isn't much you can do- you can't force your boyfriend to take a job he hates or put his dreams in the backburner so you can feel financially secure.