I walked away while writing the below, so I can see I'm duplicating a lot of what's been said already, but I'm too lazy to edit, so:
Based on his rate of progress, it sounds like your DS is on track to be doing calculus at about age 10 or so (assuming he is allowed instruction appropriate to his pace). This is still well within reach of most high schools, so I don't think you necessarily need to worry about live college classes until then. In any case, many intro level college courses are available as online or blended classes, where his face time with young adult classmates would be relatively limited.
When my sib started college courses at about that age, my mother attended classes for the first several weeks (discreetly, in the back of the lecture hall), until she was comfortable that a good transition had been made. (Of course, not everyone can do that.) I also attended some college courses as a pre-teen, unaccompanied, but with frequent consultation between my parents, program staff, and my professors.
As a parent, I have a hard time imagining my own children taking college courses at that age, but I know my siblings and I did it without any noticeable ill effect, so I'm pretty sure it can be done...
WRT science, labs are great, but there are safety concerns with young children for some types of labs, even if they are very careful, simply because the scale of lab equipment is not suited to their size. I think it might make more sense to not worry about for-credit, sophisticated experimentation, and, instead, give him access to a range of information (we used to have subscriptions to magazines like Scientific American, when we were children--wait, maybe my parents actually wanted to read that themselves?). On the experiential end, the basics of physics (other than quantum) can be picked up in everyday life observation. (I think there's an old physics teacher saying, "Everything you need to know about physics, you've learned by the age of two.") Most of biology up until biochemistry is just observation of organisms around you. Tons of chemistry happens in the kitchen. (We had a little book, now out of print, I think, called Science Experiments You Can Eat. Oh, here's the revised edition on Amazon:
http://www.amazon.com/Science-Experiments-You-Can-Eat/dp/0064460029) Lego robotics and design software and MIT Scratch are highly recommended by many people for engineering.
And don't forget, the scientific method can be applied to anything open to repeated measurement. If he understands the empirical method before college, he will be well ahead of most of the adult population of this country.
I guess I am not as concerned with credentialing, rather with feeding the interests of the child, and a solid foundation in scientific thinking.
