Hi issa2022 – this is a super common issue among couples who have dealt with or who are dealing with vaginismus. As you’ve described, many of these couples have to find ways of adapting to their inability to have penetrative sex, so transitioning toward intercourse is a big change for both partners.
It sounds like your partner is getting overexcited, and I think you have a few options. If you’re not having sex often or if he isn’t masturbating, you could encourage him to try doing so a bit before you plan to have intercourse (whether that’s an hour, a few hours, or a day before will depend on your partner’s libido and patterns of arousal). I think either trying this or just trying to have sex more often will hopefully mean he’s not so over-primed that he ejaculates right away. Also, hopefully once he gets more used to the idea that he’ll be having sex, it won’t be so outright arousing to him that he ejaculates right away.
Let us know if this works or how things are going – you have worked hard to overcome things from your side and are making amazing progress!