These are complicated decisions involving lots of emotion, uncertainty, and money. Having counseled many kids in your son's position and having interviewed applicants to Harvard University over the past sixteen years, as an alumnus, I have witnessed many high school seniors and families in similar situations.
The easy uncluttered equation is your son's academic excellence in math and chemistry + acceptance in a highly technical college emphasizing the "hard sciences" + the college's 100 percent placement rate of its graduates = a well-trained engineer with immediate job security and a bright future in a highly specialized field. What could possibly be wrong with that equation?
I am sure that your son feels pressured to place himself into that equation because his teachers and you are encouraging him to pursue this higher educational route. He appears to be highly proficient in the types of science courses that will be offered at this college and he has received an early acceptance. There is a part of him that is saying, "Go ahead and go to this school. You'll probably do well academically and you'll set yourself up for a career in engineering." But will this limited, one-dimensional curriculum will satisfy his intellectual and emotional needs over the course of his college career? I think that this simple equation needs to be re-examined by you and your son .
He is clearly not so blindly passionate about engineering that he can't entertain any other academic and career pursuits. I'm also certain that he fears your spending all that money on this technical education, only to have him say, at some point, that he wants to leave this school for a college with a more diversified curriculum. My bet is that he does not want to limit himself to this one academic focus. He wants to have the opportunity to explore other academic areas that have interested him.
I would suggest that you pay the $100 to secure him a place in this college, if you can afford to do so at this stage. That deposit keeps this door open while he considers other colleges that will give him the breadth of a diversified curriculum (and perhaps a very different overall college experience). For example, he may wish to consider liberal arts colleges with a strong core curriculum in engineering and the sciences.
There are no decisions that he will make which are set in stone. I would like to see him go to college with an air of excitement, curiosity, and a hunger to absorb new knowledge. Talking to some professional higher educational counselors as well as doing some honest soul-searching within himself will yield a truthful decision. I know that you'll stand by him and support him during this uncertainty.