Have your dad put the trailer on the scales and check the hitch weight when he is loaded. It sounds like there is not enough hitch weight with the toys in the back. You should have about 12% to 13% of the trailer weight on the hitch. He might have to move some stuff around inside the trailer to achieve that figure, then readjust the ball angle and reset the WD bars according to the specs. This should eliminate most of his sway.
03 Silverado cclb 3500 drw, D/A, Reese 22k hitch
07 Alfa SeeYa Gold 30RL,2 slides,power everything