Children's Unisex Headless Costume

What size should i buy for a 6 year old boy?

My son is 12. Wondering if this will fit him. He is about 4'10"

How small would this fit? i have a 4 year old who is obsessed with the headless horseman who wears an xs in boys. please advise. thanks!

I saw a lot of complaints about the shoulder rig being uncomfortable, is there a way to add padding?