I think even if he is telling the truth (which Im suspicious is unlikely given his form on other things) you still need say no. You have other appointments which cannot be changed etc.
He needs to tell the garage he can’t bring the car in until 10.30am or whenever. Or he can do the following day instead. The car isn’t undriveable so one extra day won’t make any difference.
i feel it’s really important for you, OP, to make a stand on things like this early on or you will struggle in the future. Last time it as wanting to drop them at 7am for a haircut. This time, a garage appointment. What’s it going to be next week? Rearranged tattoo session? Dentist? Delivery? He’s going to keep pushing the boundaries especially if you acquiesce.
This is your chance to help set the correct pattern - one where he sticks to what has been agreed.