Average time to complete a Mechanical Engineering Degree

[soliloquy]

Greetings! I'm asking here and about a half dozen other places. But, do we have any mechanical engineers in this group?

My son is in college and getting a degree in Mechanical Engineering. He has been able to manage As for the first year and we are very proud of him. He also attended during this summer to knock out more credits. He's a rising sophomore.

We had an academic advising session this morning, and I mentioned to her that his degree plan is very rigorous. I wanted to know the outcome of realistically completing this degree in four years. This morning I saw that the American Society for Engineering Education (ASEE) found that 50% of engineering students drop out or change their majors compared to 40% for other degrees. That's not my son's fate, but it's telling.  

The friendly academic advisor told us that STEM majors, particularly mechanical engineering majors, actually take, on average, 5-6 years to graduate, although it can be done within 4 years.

And apparently, there are times when some classes are unavailable in a particular semester, and it's challenging to plan ahead because classes are confirmed a couple of weeks before registration opens.

Anyone with a STEM degree has ideas on this? I'd rather him absorb knowledge and apply it over the course of 5 years rather than cram and not have really been able to process and absorb this information in 4.

Any thoughts?