How can I help my husband overcome shame about his small penis?

For you or for him? He's probably quite self conscious about it & his ability to satisfy you & thus needs assurance that other aspects of intimacy are more important than inserting a large penis.At least 50% of men wish they had a larger & majority are within normal limits for size.He should be able to bring you to a sexual climax by stimulation of clitoris if necessary. Mental & spiritual relationshid more important.
Teach him. Other things exist in addition to the penis that can pleasure a woman. Show him what hands, fingers, and a tongue is for!