Hello.
First of all, you need to understand that the position you have mentioned is not conducive if you are trying to conceive. There are chances that the seminal fluid will flow out soon, before it reaches the inner parts of the cervix and then inside the uterus. You need to have sex in a position where he is on top.
Second, your husband has probably conditioned his sensory system for the stimuli coming from masturbation. That's why intercourse doesn't help him achieve the excitement. There's nothing wrong with you. It's just that he needs to re-train his sensory system.
Since you hardly have much time, you could use some short cuts. If getting pregnant is the priority at present, he will have to adjust to the need and change a few things about his sexual preferences, including the position.
Wish you good luck.
Regards
This post is for the purpose of providing medical information and is not a substitute for a face-to-face consultation with a doctor. This post is not intended to give or rule out a diagnosis, create a doctor-patient relationship or replace an existing one.