I live in Japan and have bilingual kids, as do most of my friends.
I've seen all kinds of situations when it comes to moving kids about from one language system to another--not just kids entering Japanese schools at later ages, but also (say) German-speaking kids being put into an English speaking international school when their English is minimal, or expat friends of mine deciding to go to France and put their kids in French school when none of them have much French
Welsh is a little different, because all the kids at the school will speak English as well as Welsh, I assume. That will make the social side of things easier--but on the other hand, it may also slow down acquisition of the language. If kids (naturally and understandably) switch to English whenever they speak to him, his acquisition of Welsh will be slower than it would if he was popped into a Korean or Spanish school where the kids didn't speak much English.
I've seen Japanese parents putting their kids into international school with no English, and getting all cross about the slowness of their child's English progress--the reality is that most of the kids at the international school probably speak Japanese if they're growing up in Japan, and tend to switch Japanese when the newbie kid with little English speaks to them. These kids can learn, of course, but it is definitely a slower process.
Do Welsh speaking schools have any separate tracks for kids who come in at older ages with little Welsh? If he can do his academic subjects in English while doing Welsh study at the right level and doing his social stuff in Welsh speaking classrooms and environments, that's perfectly doable. If he's expected to do all his academic subjects in Welsh AND SIT EXAMS IN THEM AT SOME POINT IN THE NEAR FUTURE, that is a bit different.
By secondary level, kids are doing fairly serious work in their languages--learning history and chemistry, writing essays, doing complicated word problems in maths. Trying to get to that level of fluency in a second language that he doesn't speak much will take time. Does he have time? Do you trust that he is really going to nail all this in enough time to get some decent scores in whatever qualifications he will go on to take? Is he the type of boy who is prepared to really really go for it and do all the hard slog of mastering a language to full proficiency in a couple of years (including literacy, learning a big vocabulary of specialized, scientific and high-level worlds) while simultaneously being chill enough to be OK with being the struggler in the class for a long time? A few kids can cope with this. Others (once they have acquired some initial superficial fluency in everyday language) start to plateau and get frustrated. Many get depressed or lose confidence, because they have this experience of going into school and being the weakest person in the class, and you have to be quite resilient to be OK with feeling like that for months and years.
Mumsnet tends to be full of people going on about how kids are "sponges" when it comes to languages. But getting to "everyday/playground-level fluency" in a language, is very different to the kind of mastery that you need in order to write an essay in the same language and get a decent grade. It takes many years to get to that level.