Brought Buttons outside since its a warm day. Should I be worried about sunburn on his ears?

He's just a very old guy and his hair is thin on top of his ears. Its a nice warm winter day so I thought I'd take him out with me while I do some chores. He seems to be enjoying the sun but I'm a little worried about those tjin spots getting burnt. Should I restrict him to a shady spot? Bring him in after 20 mins?