I dunno. I looked up Jimmy Dean Sunshine Man and there's a lot of pictures. I'd think the material would be the easy part. The triangles could probably be made from something similar to Styrofoam and covered. I might just start with a big (huge) yellow "t" shirt and then one that fits him (your son)...cut out the arm holes in the chest of the big one and sew the smaller one inside after pulling the sleeves through... then sew the waists together making a pocket and stuff the living heck out of it with packing peanuts or foam through the neck hole (big shirt) and maybe put a couple buttons on it to keep 'em in.
Take the covered triangles (maybe a couple knee pad cushions cut diagonally) and sew em on. If it doesn't have to be perfect maybe some of those swim noodles (yellow) pushed through small holes on the big shirt?
Maybe a swim noodle in a circle to make it look round and then attach the triangles (Hot glue) to the noodle and poke 'em through the big shirt for the points?