Dianna, teaching an artist's kid to be a good model is an essential!
I get around some of that by first of all using a long lens (my workhorse is a 70-210) so that I'm not on top of the person and can allow them to relax a bit, and secondly making it very clear that the responsibility for a good photo is mine, not theirs, and I just need them to ignore me and go about their business. Not to say that I follow my subject around the house or yard, although I sometimes do that with small kids, but you can place the subject in a location with decent lighting and give him or her something to do and then just snap away. I take multiple shots, including closeups and more distant views so that I can get in all the information that I think I'll need. I also use fast film: Fuji Superia 400.
I agree with Mike about the clothing; it's easy to change colors and patterns but harder to change collars and sleeves and the way an article drapes.
I meant to tell you that you have a very handsome son and I don't blame you for wanting to paint him. I would, too!