Because he will (I think and this is why I’m asking for advice) be expected to answer questions from the POV of a Catholic
As others have said, that's not how this exam works. You can look up exams.
Also, I've known very academic kids who fail RE - like getting 8s and 9s in all other subjects and then a 1 in RE. It had nothing to do with whether they share the faith(s) being tested, and everything to do with having family and friends tell them that it's a pointless subject, that they can just leave it off their CV and to focus on other subjects instead.
have found a good online course for him to do so he won’t miss out educationally. I’m wondering if anyone else has experience of withdrawing/not withdrawing in this situation, from either a teacher or parent POV
If you want your child to do an online course during that time, you'll need to discuss with the school about whether he'll have access to a computer. That's not automatically a given and they are not required to supply that. I would not rely on that being an option - IME, it's really unlikely.
As a parent of a child who was withdrawn from RE while at a faith school that was the only accessible option, I think doing it based on the school's website and ideas of what you think the exam will be like is a premature jump.
There will be a lot going on that isn't on the school website, and you can request a meeting to discuss your concerns - if you withdraw, they'll likely want to arrange one first.