According to the Legacy of the Force series, he always wore durasteel because his best friend (imagine that for a second) tells him he doesn't understand why he would still want to be wearing his old inferior durasteel set when they can make him a new set out of their new iron cache. He even goes so far as to tell Boba that if it's for sentimental reasons, they can "paint" the dents and scratches on it.
He does eventually give in and accept new Mando iron armor complete with neck and the full Jango-style leg plates.
As for multiple helmets, it is written in several sources that he carries multiple sets aboard SLAVE 1. That deceased "nobody" Jodo's now being one of them.