Hi, I assume at one point you two had a sex life together. Questions you can ask yourself which may provide answers for you:
When you had sex together, who initiated it and how? Did he enjoy having sex with you? Did you with him, and did you let him know how much? Did you notice anything going on either with him, you, or your relationship at the point where sex tapered off?
Is there intimacy in your relationship? Do you express feelings of love for each other? Is there touching of any kind (hugging & kissing, hand holding, arms around each other)? Do you spend time talking together and sharing? Do you enjoy activities together?
Your answers may paint a broader picture of your relationship and help you uncover ways to become closer. You could try to take your relationship back to a point where you were doing well as a couple. Recreate that environment. Maybe take extra care to listen to him and inquire about his day. Tell him how much he means to you. Woo him. Cook a nice candlelight dinner and tell him afterwards you’d like to give him a massage. If he doesn’t respond positively to several attempts like this, it will present a natural opportunity for you to ask if something is wrong.
And perhaps something is wrong – either with his hormones or male parts. You said he discussed this with his doctor, but you don’t know what the doctor said to him in response?
If he trusts you and opens up to you, I’m sure this can be fixed. If he clams up and shuts you out, you may have a bigger problem.
Best wishes to you, Flower
PS If there's a way for you to learn more about his parents' marriage, that may provide valuable clues for you. What he observed in their marriage may be affecting yours.