I can right my P19 with ~310lbs and it is a bigger boat with a taller rig using the standard righting line. I usually sail with someone in the ~180lb range and i'm a lightweight @ ~130.

You *should* be able to right just about any 18ft boat you are looking at with 300lb of crew weight.

edit: so you and your son are around ~265, definitely on the light side for the larger boats. N5.2 or p16 would probably be better with that weight.