Seems to me your company hired one guy to do a whole team's work. And since that guy didn't complain the first time, they threw another project at him.
They will keep doing that until you start complaining or you quit.
Being required to constantly shift programming language, do backend, frontend devops and QA is not normal. It's what bad or abusive employers do to new hires or to people that let them.
From your previous responses I understand you do QA Automation.
As a fellow QA eng. I understand your struggle. I was also tossed from product to product and required to learn products at neck-break speeds and test them.
I made it a point to never work on more than 2 projects at the same time and if I did work on more than one, I tried to reuse as much stuff in my testing frameworks as possible.
If management dictates how you do your job, what languages you use to write your tests, what frameworks etc, then you are not a QA, you are QC, and they should tell you exactly what to write in those tests, you just run them.
If they strip you the choice of using your own experience and force you to learn and use tools just because they said so or because they think it's a good idea, that's not a company you want to work for in the long term.
I've met some code factories that do that, most of them outsource work. They provide no real value to you or their customers, they just implement what they are told with the minimal amount of effort and just move on to the next client. You being a QA are most impacted by this type of doing business, because in theory you should serve as an advocate for the client.
Not all companies are like that. Most will require you to constantly learn new things to provide value in your work but not all will ask you to do the job of an entire team.
Answering your question: just tell him you are not comfortable with doing your job his way. Tell him you are getting burned out and ask for reprieve. And start looking for a better employer if the current one is unreasonable.