I think you need to ask if the trip is mandatory, and if so why? (I don't think schools are allowed to make such tings mandatory). That said, and without wishing to pry, it would be a shame for him to miss out on such an opportunity which I imagine might augment his studies and so it is worth exploring his lack of interest, if it is subject related perhaps history is not for him, if he is otherwise keen what can be done to ensure that he is able to participate in the subject fully? As for law, wow! him for already knowing that is what he wants to do.
There are no GCSEs required to be a lawyer (or indeed A levels although a strong analytical bias via an essay subject helps). OH is a lawyer, no history GCSE, science A levels and Masters and then a conversion, on finally (sigh

) studying his conversion he was snapped up and sponsored by a magic circle firm precisely because of his 'unusual' background. That said he has always been somewhat of a wordsmith and this was 20 years ago.
To 'help'persuade him about the trip it might be worth emphasising how much travel/time away/going outside their comfort zones top lawyers have to do, indeed how much they have to do that they do not want to at all, especially in the early years

.
Good luck