They can in fact knot, I've had experience with several fixed boys in my time and there were never problems getting the knot to grow. Unless trained to mount or fixed after learning that humping = fun though, they may not be very inclined to penetrate in the first place, so stimulation will have to be manually done.
I can attest to the lack of cum though, that is true. Their body do the movements but unless there's a bit of urine already in their system, the contractions will be running on dry plumbing.