MFL stands for modern foreign language.
There is a good chance your child will do Spanish in the US system, but it is not guaranteed in middle school.
The most common languages in schools here are French, Spanish and German. You will want to make sure your child gets to continue with whatever they have been doing, or pick up a language that is new to everyone in Y10 (some schools add in a third language in Y10).
Modular GCSEs means that they take exams up to three times a year over the two year course. Right now, Y11s can take fully modular courses with lots of resits should they not do well.
Y10s can do modular courses but they are limited with resits and have to take 40% of their qualification in June of Y11.
Courses starting next year are linear, which means the only exams are taken at the end of Y11 for 2 year courses.
This is the situation in England and Wales. I don't know if it affects Northern Ireland as I think their Education policy is devolved.
I don't think you really need to be on top of the exam system for your return. There are a few things that you need to be aware of - how specific course differ from the US.
Most subjects will start their GCSE work afresh and not really require too much prior knowledge that he won't already have. For example, he should be fine in English and Maths. To be sure in Maths, make sure he does accelerated or advanced Math in the US system rather than 'college prep'. He should be fine in Biology and Physics, but may not have had any Chemistry. History would probably cover different topics (this is the same for a move within the UK, as it is often up to the interests of the teacher), but he'd be behind in Geography (which is a small subset of 'social studies'). I think he would probably be fine in Expressive and Performing Arts, and be an asset to the school in terms of extracurricular music and drama.
Kids are resilient and can cope with most things, but it is a big ask to transfer at the start of GCSE courses, unless in a small school where he can seriously get individual attention. I just don't think his results will be representative of his potential. An option is to go into Y9 instead of 10 to acclimatise.