The doctor. most likely, is referring to the settling process. In this process, the muscles are still adjusting or on the process of healing so he is experiencing some back pains due to this. Once the muscles are settled in place, he would feel normal again.
you could use refusual strategies such as assertivley denying or giving another solution or excuse like "oh why don't we do something else that donesn't sound very much fun to me.
