Enjoy the journey this next year. You are asking great questions.
Help him find opportunities to investigate different careers to see what he may be interested in. Nothing needs to be set in stone, but having colleges with lots of options is best for undecided. As folks have said, it is easier to transfer out of engineering than into it sophomore year. So if kind of interested in engineering when applying, most would apply to engineering.
When you look at college tours, find some close by first and try to find days when colleges may be having more in-depth tours of different departments. (More common at less prestigious colleges but trust me, this helps). Even if it is not a school high on his list, being able to see research projects and course options helps them figure out if that major is something they really want to do. Have him look at required courses, flexibility, and where double majoring may be easier based on his varied interests. Most engineering programs leave little room for even minors, but other schools somehow build in more flexibility he may enjoy.