The term 'crutch' is used to denote the fact that one relies upon one thing to avoid and make a second harder thing less difficult. In your case he is directly referring to your continued use of the splint. What he is saying is that if your wrist is healthy enough to use you should be doing so; that, in this case, I read this to mean he feels you are using the splint as a 'crutch' to avoid pain. The longer you continue to do so the slower the wrist will heal - if it does at all. That is also what he said about working through some pain; to pushing yourself beyond it.