There is no way for anyone here to say whether or not his shaking has anything to do with the dog (I'd suggest a trip to your family doctor in that regard probably wouldn't hurt).
But as for the rest - no, she shouldn't be sitting on him. She probably shouldn't be sitting on the couch either (unless it's at your express invitation). Dogs are dogs, not humans. And if you treat them as the latter, then it does tend to lead to problems. It sounds as though you need to take a little more control of your household here, and set some clear boundaries for the dog. Interactions between small children and dogs should always be supervised, of course, so it's a fairly simple matter to ensure that if she's taking a liberty or two too many, that you instruct her otherwise (but don't forget to reward her compliance).
LOL - small edit to add that I was assuming here that you were talking about a child... If the 'boy' is a dog, the above all still applies minus the bit about a family doctor
