lack of sex drive in my boyfriend

My name is Debbie and I am 44 years old. My boyfriend is 51 and we have been seeing each other for 9 months.
We both have never been married. We do not live together but see each other every Saturday night and also during the week.
We had an ok sex life at the beginning of our relationship (he does not like giving me oral sex so I have adjusted to that) then the past 4 months he has no desire for me. He is a doctor and under lot's of stress so I know that can affect his desire. He is also often tired.
I tried to talk to him about this and he says it just makes it worse to talk about it.
Last night, Saturday, I tried to initiate sex by touching him and he laid on his back and did not reach over and kiss me or touch me. I almost got an erection out of him then it went soft. I was very upset and told him he does not give me any affection and I can not keep on going on like this.
If his erection is gone he can still try to give me pleasure but he doesn't, he just goes to sleep.
I am the one who does all the initiating of sex and he just lays on his back and doesn't do a thing to help.
We have a great friendship and we really like each other and respect each other. Our bond is going stronger but I would not say we are in love yet.
I want us to have a sex life and I can not make him want me. What can I do to help improved the situation?
Your advise would be appreciated,

Thank you,

Debbie