I hope you don’t mind I’m giving you a male perspective on this?
What your therapist told you is correct – a large percentage of men will have issues ejaculating from vaginal contact if they never had this kind of stimulation before.
You can un-learn that though (I’m not even sure if this is even a correct English word). The key is to let him to stop masturbation completely, and to stop stimulating him manually/orally as well. So he basically can’t have an orgasm anymore for a while. Nothing. No exception. You can already start doing that now. He’ll probably get a few nocturnal emissions, but that’s okay, that’s his body getting rid of the excess semen.
When he’s allowed to penetrate you, ONLY do that, so still no masturbation or stimulation. Even if he can’t finish because he gets too tired or has troubles keeping his erection, DON’T stimulate him to an orgasm. Wait a few hours and try to have sex again, or do another attempt the day after.
You will see that his body will start to make the switch in a few weeks. The more you try, the easier it will be for your partner to ejaculate from vaginal stimulation. It’s important your partner doesn’t resume masturbating for at least a few months though. Good luck!