I wonder if having been out of the country and perhaps not linked in with other school muns it might be hard to know how/where to access lessons for a boy who isn't the right age/skill set for typical group lessons.
You have a few really good options.
The best is to take him yourself, either with the book recommended above or YouTube videos. He will pick up the basics quickly at his age; this is something you can do yourself starting today.
Look at: state secondary schools, grammar schools and private/public schools in your area that have a pool. Find out who runs the lessons. Call them and ask about 1 to 1 for a Yr 8 complete beginner.
Look at gyms (Nuffield as recommended above) or hotel health clubs in your area. Loads rent out the pool for lessons. Again, find out who runs the lessons, call and ask about one to ones.
There are also private companies that offer lessons; again call them, explain the situation. Even where we are, these run in very rural locations eg in a pool on someone's farm!
We have also been swimming, seen teachers doing the lessons in the pool and asked if they have time themselves or a recommendation for a one to one teacher.
We also live in an area where swimming lessons get very booked up and have had success with all these routes. The best option though would be just a few one to ones supplemented by just taking him swimming to practice, bit like learning to drive!
Good luck!