I'm not sure about "techniques", they all come across as bogus to me. When I was a kid and my dad tried "techniques", he simply came across as a manipulative twit. This kid will see through that instantly. This situation screams to me "be genuine, don't jump into some talking head's pet theory, including mine".
I am of the opinion that our kids learn as much from how we handle failure as how we handle success.
Don't try to convince him of the value of middle school. There's a vocal group of middle school teachers who feel it's wasted time. If a significant fraction of the teachers feel that way, you wont convince the kid otherwise.
According to Chris Hogan's book "Everyday Millionaires", most people who retire as millionaires earn their money the hard way (are not born to it), never break the 6 digit salary ceiling, and never averaged higher than a B in school, so the kid has a very strong point. However, he's turned the point into defeatism instead of looking at what it does take to succeed.
Being a complete stranger on the internet, with only the information here,, I think the first thing that needs to happen is some serious relationship building (insert ironic chuckle - this is coming from an autistic person with brain damage, for whom a person that is not in the room might as well not exist).
It is possible that the relationship may be already irreparably damaged, depending on the father's take on the insult. If not, some one on one heart to heart time would be in order.
There is an old Chinese proverb that is often misquoted in English - either you think you can, or you can't. The English version is usually expressed as a guarantee of success of a person only believes in themselves. The original only guarantees failure if a person does not believe in themself. This young man needs to learn the difference, and IMHO his father should be the one to teach him, or possibly this could be a lesson they learn together.
Spend time together, as partners. Study people (together) who failed then later succeeded, and learn from their examples. People like Buckminster Fuller and Dave Ramsey, The kid has shown the beginning of adult comprehension, channel it towards long term success by making him a partner in the journey, not a bystander.
My own son is aware of my weaknesses (he explains it as "that part of his brain is broken"), nevertheless we have a relationship of deliberate mutual respect.
As avid anime (Japanese Cartoons) fans, we occasionally adopt some Japanese mannerisms, and I will address him with the -Kun honorific, indicating someone younger who is respected, rather than -Chan which is used to address very young children. Occasionally he will refer to me as "Sensei daddy"
You can see, acknowledgement of weakness does not need to diminish respect.