Screaming going in the car seat

My 15 month old has started screaming when going into the car seat if he’s even remotely tired. Hysterical, will stand up, can’t get him sat back down. He goes all stiff! I’ve tried bringing him back out the car to calm down and try again (he usually just smiles and is happy as soon as I do this). Does anyone have any tips? I’ve tried singing, distracting with a toy, putting some music on in the car