The use of this kind of strap-on should be instinctive for the man, who, without having to worry about premature ejaculation or impotence, can work for hours pleasuring the Wife. He himself will only experience vicarious pleasure. Furthermore, after being accustomed to a larger phallus, if he ever tries to have intercourse with his actual penis, he will feel even smaller than he actually is.
In order for your partner to truly feel comfortable in his own identity as fully 100% receiving male and completely give himself to you. You need to not only encourage that behavior, but let him know that being penetrated by you is OK.
If he has a small dick, or if it is smaller than your strap on, remind him how small he is compared to you, and encourage him to think that his cock is small because it is only meant to pee and not to fuck.