DS is 18kg and 4.3 and has just moved into a HBB: this one. But when he fell asleep in it today he was slumping forwards and flopping to one side. I woke him up as it looked awful. Is there anything that I can safely use to keep him upright in his seat? I've seen this kind of thing. It seems ideal but I am always wary of "add ons" that could potentially make the seat less safe.