I agree with animenerd, rigid foam (insulation variety) can be glued and layered and painted to create a hard shell, while remaining very light. No ribs or frame needed!
Secret Pro mascot builder trick: get a helmet that fits comfortably, whether football, construction, baseball batting helmet- whatever. make or acquire a rigid cardboard tube. adhere said tube to the top of the helmet. adhere the inside top of the costume to the top of the tube. ta da! tall costume with minimal support structure/ complications.
as his arms will be inside anyway, he will be able to lift the helmet off his head for the needed breaks.