Yes, the Poly's volcano slide is a tunnel. When you climb into the opening, you cannot see where it ends, nor can you look up and see ski. It has 4 bends in it, where you slide to the right a bit, then the left, right, left, and then woosh, into the daylight and water. It is illuminated, with a slight red glow to represent the inside of a volcano. The inside is high enough that many children can ride sitting up (DH and I had to lean back, and many older children rode down flat on their backs for the added speed).
My DD7 and DS7 are not thrilled-seekers by any means and aren't particularly thrilled with dark rides, and they loved loved loved this slide! However, you know your DS best, and I'd say if he doesn't like the Chuck E. Cheese or MacDonalds-type closed/tunnel slides, then this might be a no-go for him.
It's one person at a time on the volcano slide, so although you can ride directly before or after him, you can't together.