Erection Issues. Week dosage of vit d3.

My husband is stressed because of work for the past 1.5 years or so. I guess this is the reason for his lack of sexual desire. We are planning a baby now, it's been 5 years of our marriage. We both are 33 years of age. The problem is that he is not able to maintain  the erection for the intercourse, infact there is no proper erection also. and this is frustrating for him. I advised him to see a doctor but he feels shy and awkward to see a doctor for this issue. Would you be able to help, please! Though a little irrelevant, but just want to mention - my husband was having severe back and neck issues for the past two months. It's fully cured now with the help if physiotherapy. But he is taking his once a week dosage of Vit D3.