Boyfriend is rude to his parents

Just seen a post similiar to this and it's got me thinking.. my bf (he's 23) lives with his mom and dad and when i go round i notice he's always rude to his mom. She does fuss a lot but i see this as her taking care of him? But he blantly ignores her and starts chatting to me instead and he often snaps and says shut up under his breath. He acts like a sullen teenager with them. Is this a red flag?