Please describe your flying abilities (and your son's) in much greater detail.
The only thing the LT40 and the Hog Bipe have in common is they are both airplanes... the flying characteristics have little in common.
While I agree that the Hog Bipe is not 'tricky' compared to other planes in this class, it is going to do absolutely everything much much quicker than your LT40.
The Hog Bipe is a great first biplane (it is not tricky) but there is a difference between being a great first biplane and being a good second plane.
Without seeing you fly it is hard to recommend the Hog Bipe as a second plane.
Now having said that... if you are flying the pattern inverted and doing rolling circles with the LT40 then the Hog Bipe will be a piece of cake to transition to.
Talk to your instructor and see what he thinks about making this jump.
From afar I would recommend putting at least one more plane between the LT40 and the Hog Bipe, an Astro Hog or a Four Star would be a nice step up.