Does everyone feel lost studying engineering in college?

I think a lot of people feel lost studying engineering. I too had a lot of hard to understand professors and some of them did seem to only be there to teach in order to do research, so unfortunately (or fortunately depending on how you look at it), this is a pretty common experience. I remember going from being a 4.0 student my freshman year and being one of the few to pass one of my classes sophomore year with an A, and then third year rolled around and suddenly nothing made sense anymore. I'd be able to do well in homework, but like your son, when tests rolled around, I'd feel lost and would often do poorly. Pretty much all the seniors I talked to let me know that third year was the hardest of the four, and many of my classmates agreed. Now, I didn't study Aerospace engineering, but I can say that over time, things did start to get better. I was able to take more elective classes that I had more interest in than the core classes I was required to take. Senior year was amazing, I was only taking elective classes and working on my design project, which really finally solidified a lot of those confusing concepts with real life applicability, so I wouldn't say it's time to cut his losses, though I also wouldn't recommend staying in a major he truly hates either if that becomes the case. 

As for what is expected on day 1 of the job is really dependent on company and your son's level of experience (how many internships, projects in college, practical skills, etc), but I can at least talk about my experience. Day 1, I wasn't actually expected to know much at all. I was trained by a mentor who sat with me and helped me understand the work that I was doing. I was given tasks that represented my experience and then given harder tasks as time went on. It was about 4 months in that I started working completely on my own. 

Also, often with Engineering jobs there isn't a one to one correlation between your job and what you learned in school. Managers are looking for people who can learn, work hard, and push through adversity, which are three of the most important skills that come out of any engineering degree. I rarely actually use my degree, which will be different depending on the job you get, but I use those skills that I learned from getting an engineering degree everyday.

Some advice I'd give is if the professor isn't helpful and the TAs are equally so, reach out to fellow students. See how they are doing. Some students will get some things and other will understand other things. My third year, I started working with other students on homework and studying for tests and things started to make a lot more sense. I'd always been a lone wolf when studying, but working and collaborating truly made the biggest difference in how I viewed getting my degree. Collaboration is a huge part of the job, so those study groups really helped me as time went on, well into helping with collaboration in my job.