D.,
This is one of those times in life when you are presented with a "life lesson opportunity". Explain to your son that you are concerned that there is more to visiting a college than finding out what the campus looks like. A visit gives you the opportunity to see the area around the school, to find out if it is in a bad neighborhood, or if there is nothing to do for fifty miles, or if there's no place to park a car, but no public transportation, all things that will affect his quality of life while at school. Visting the school let him get a look at the other students, and find out if it is the kind of community where he will fit in well and be comfortable. If your son is a real straight-arrow type kid, and half of the student body has purple hair and a tongue ring, that school might not be the place for him, and he isn't going to see that in an on-line tour. Choosing a college is like moving to a new town for nine months per year. How mad would he be at you if you sold your house, moved him to a new town, and never even bothered to see the place first?
Now, once you explain all of this, it would be wonderful if your son said "Oh, I see what you mean, mom, let's schedule those trips right away!" But that ain't gonna happen. He's going to insist that he do it his way, because that's what 16 year olds do. Here's where the life lesson comes in. Let him. Let him look at all of the tours online, let him do it his way. BUT, he's stuck with what he picks for the first year. If, after a year, he feels he chose wrong, then he can transfer, but he's GOT to stick out the first year. This way, the responsibility for his happiness rests on his own shoulders, but the risk is minimal, because he can always transfer to another school. If he chooses badly because he didn't have enough information, and ended up miserable for an entire YEAR, then perhaps the next decision will be made a bit more thoughfully, yes?
D., you might also think about whether you are putting this on him a bit early. Most schools don't require applications until December of senior year even for early decision. He's got a year before he has to make up his mind. Maybe it's that causing his resistance?