mmz I've been in this position with ds1, and the opposite with dd! Ds1 really did not want to budge from his present school. He has been happy there but it wasn't the right fit for A level socially or academically, (possibly because they were in a very fixed academic mindset, and ds should never have attempted 4 A levels, and it has totally screwed up his choices and he has ended up in a muddle. Also the school while supportive is geared up toward more academic pupils who are doing Oxbridge) Ds however was adamant he was staying. Dd on the other hand is keen to leave her current very good school to go for a more exciting alternative, which iI don't think will suit her all.
However, I think the best thing is to press on with the interview, and see how ds feels when the dust settles, and when he meets some of the other people also going for interview, visits school again, you may find he feels a bit more "at home" there and beginning to consider it.
There are always new people in every Sixth Form, it is not the same as the lower school even if you think it is your familiar school, and tbh whatever is wrong with your current school may be quite different in Sixth form and vice versa. I am hoping dd will have changed her mind by the end of this year, and want to stay (state school so no deposit) but I am not ruling out that the private school she has set her heart on will still be in the running up until the time they ask for first term's fees, at which point I am going to have a serious talk with her, and we will decide then. There is nothing worse than not having a plan for next year, and possibly that is what your ds wants, Plan A Plan B. ds1 was determined that there was going to be no deviation from Plan A, and tbh it was very exhausting, not knowing from June to Sept what was actually going to be the outcome.