Hi all, thanks for the helpful comments.
@PokerFriedDips no option for classical languages - it's French or Spanish! Of the two, Spanish would probably be least challenging.
Re the SEN dept, he was flagged to them on entering in year 7 but has never had or needed any input, so they probably have very little/ no knowledge of him at this point. This has all been good from my point of view, as he is doing brilliantly academically, has a group of close friends and has never been bullied (my main worry). I've been more focused up until now on him being able to do everything as normal, it's only now really I'm thinking that it could actually disadvantage him in his academic attainment so I want to think strategically about it.
To clarify @clary, he has a speech sound disorder - he had to be painstakingly taught to say the majority of sounds correctly, so as a young child he could not say plosive sounds and would make the substitutions you mention eg 't' instead of 'p' or 'd' instead of 'b'. He can now say all of these sounds fine. He still struggles with 'ch' and 'j' and was taught to say these by saying 't-ye' and 'd-ye' really quickly - which it still sounds a bit like! His vowel sounds are often just slightly off, which can make him hard to understand, and overall his speech is a bit 'slushy'.
It's reassuring that the marks for pronounciation are relatively few - he would probably be ok in terms of his vocabulary and understanding grammar, he is currently in one of the higher ability language classes but my impression is they do very little speaking, and certainly no actual speaking tests until year 10!
If he was really keen to do a language, I would definitely want him to have the opportunity and would support him to do so despite all this, as I don't really want him to think he can't do something. He himself is not hugely self conscious about his speech and has not given this as a reason - his reason is he doesn't want to do a language and would prefer to be able to choose subjects X, Y, Z....! However, given he doesn't want to and there would probably be some additional challenges which would require adjustments and for him potentially draw attention to the issue then I am very happy to put the speech issue across to school as an argument.
I have not directly approached the SEN department, my approach so far has been to head of year and options team. I don't know if worth contacting SEN to support my request that he not be required to do the language, as I suspect this will probably not be their decision and will be affected by timetabling - obviously if he ends up doing one I would have to in order to ensure that he has any reasonable adjustments he needs. If he were on the other (less academic) pathway then a language would be optional but he would only do 8 GCSEs and would have extra maths and English - he absolutely does not need this, and I wouldn't want him being disadvantaged on this pathway as he is more than capable of doing well in 9 subjects. Historically (ie in last few years) the school have also had a little bit more leeway on this so I feel if they have no flexibility in his year group this is somewhat unfair.