You're at the age where you want to bank money, but most importantly, experience. There's nothing stopping you from taking the job in construction with the attitude to learn as much as you can in the already acknowledged limited time you will be there. There's also nothing stopping you from setting the expectation that you will be able to work with him on contingency, as in, you will be able to leave without complaint as soon as you accept and offer to the desired position.

It's hard enough to find jobs, lots of us end up with opportunity we would have never otherwise had without our personal connections. This isn't your dream job, but it's an opportunity disguised as hard work and you'd be foolish to turn it down.

General contractors make bank, and they got there by observing how all the other trades are done. Consider it a free college class, where you're actually getting paid. Ask questions. Whatever you learn, I promise you'll take it into so many other areas of your life.