It's always best to keep a young animal on the diet it is used to for a week or two while it settles in, then gradually swap over. So you really need to find out from the current owner exactly which food (including fresh veg, pellets, grass, hay), how much and how often they are being fed. A new pet already has quite a lot of changes to cope with, without risking gut issues from changes in diet.
I found that our most recent bun didn't stay on junior pellets for long (maybe a couple of weeks) before he decided the regular adult ones were nicer, refused to eat any more despite them being offered, and he swapped over with no issues. I just made sure that he had access to more than the adults had as he was growing rapidly. I now have a big bag of junior pellets left - so my advice would be to just get a small bag and see how it goes.
Whatever he is fed, he should still have plenty of hay and more pellets than an adult. Watch his poos to see if the balance is right. If you see any uneaten caecals, reduce the pellets slightly. Introduce any new food types gradually. It is unusual (in my experience) to get a proper diet sheet with a new animal - so treat all veg as 'new' at first unless you are certain that he has had them before.