As the parent of a kid who had low tone, I think it's ridiculous to make a kid always color sitting at a table for their posture. My kid built strength running, jumping, climbing, doing the monkey bars, swimming, etc. She had to fight to be able to do all of those things. Our PT encouraged us to let her be at home and not interrupt playing to correct her position, and that was for a kid with a diagnosed issue. Your husband is being way to controlling.