Hes a one year old German shephard mix I got at the pound. He doesn't seem to know how to play. Unless the toy has food in it forget about it. I've tried different toys and sometimes he gets into it but then doesn't bite down  to tug or looses interest in the ball and leaves after 5 seconds. He just doesn't seem to understand play at all. What's your advice? I really want a dog I can play with.