Coping With Erectile Dysfunction by Michael E. Metz: How to Regain Confidence & Enjoy Great Sex https://www.amazon.eg/dp/1572243864?ref_=cm_sw_r_apann_dp_HZDPSH208XKJMZXC5D1Y
I might also recommend you read "Coping with Erectile Dysfunction" by Metz and McCarthy.
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It's written more for men, but is a couple's book and they are both therapists with PhDs specifically addressing this issue.
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It might be helpful to read and then let him read it as well.