What are his doubts and why does he have them? I mean - doubting the "Joseph Smith story" is a bit vague. Try to get him to be specific; does he take issue with:
A 14 year old prophet? (What about Samuel in the OT?)
God and Christ appearing together? (Look up Stephen in Acts)
God and Christ as seperate beings vs the Trinity? (
LDS.org - Topic Definition - Godhead )
I could go on, there are dozens of specific doubts a person can have. My point is that your husband isn't going to find his answers if he doesn't ask the right questions. Once you and him know specificaly where his doubts are, you can begin to focus on resolving them.