Yep, he can get possessed, but luckily he has a secret weapon. Since, yeah, he’d definitely too small for a tattoo. Before leaving the motel in Schism, Walt gave both brothers a ring with an anti-possession symbol burned into it. So Sam and Dean, so long as they wear the leather rings, can’t be possessed.

So far, Dean has only been knocked out and tied up with Sam around. This will come up in the future, though. Good ways for Sam to help out big bro would be carefully talking Dean through an injury if it’s too big for Sam to handle, keeping Dean from slipping into sleep when he might have a concussion, even taking a bullet out of a wound. His hands are the perfect size to slip in and grab it.

Sam hasn’t hurt Dean yet, either physically or emotionally. The closest is Dean having to see Sam afraid of him after the events of Taken. Even Sam’s hurt by those automatic reactions, he just can’t stop them.