You can try bell training him. Hang some potty bells on or near the door that you will use for him to go outside. Ring the bell every time you go out for awhile.
Then help him touch his nose or paw to the bell every time you go out for potty. Praise him each time, treat too. Soon he will start ringing it on his own.
There is a search tab up in the blue bar above this text box. Type in potty bells or bell training and you will find a lot of info about it.
I trained my Yorkie with these and she caught on very quickly
