Ellen, persuade your son to finish his degree before anything else. A good degree speaks more for a person than any commercial licence. It also adds another dimension to you as a person. Nobody wants to know a one trick pony and somebody with a background is more employable. Only after the degree is in the bag consider the next step.
But as to who to learn to fly with, that is really six and two threes. Personally, I really dislike CTC. Over the past few years they have elbowed their way into to the market and have sold their graduates short. Finishing at CTC almost certainly means working for nothing for a while but, for lucky ones (if you call working 900 hours a year lucky) you will be working and be in a position to make your own way. Oxford has an excellent reputation earned over many years and has turned out good quality pilots.
As for exams, I'll hazard a guess they won't be an issue. A nice fat cheque book generally guarantees a pass. Regarding cadet schemes, these vary and this is where the graduates score. During the interviews, the person will be questioned and the more of the person there is to question, the greater the chance of success. MPL schemes are good if you can get on them. It's a long road but probably more useful for airline flying, especially as the graduate of the scheme has to be employed by the training organisation. You will be told during the application process whose and how many selection exams will have to be sat.
Final word of warning. The good days of aviation gave gone. I have between two and seven years left (I haven't made my mind up how long to work) and really enjoy my job. But I'm lucky. Probably because I'm working for an enlightened legacy carrier. But I know (too) many people who do not enjoy their jobs. They are being worked to the very limit of the their law, they work for despicable, rapacious outfits (non-European) and have to toe the line. They have no job protection, no union representation but are well paid and fly very big, new shiny aircraft - but hate every minute. You can't do that for your working life!
And let's not forget, the most difficult thing is to land is a paying flying job, not the aircraft itself. There are too many people with licences looking for a job who are prepared to work for nothing. Then, when you have a job, you are expected to work your backside off and do exactly as you are told. It's not a job I'll encourage my kids to do.
Sorry to be so horrible, but my view is one from the inside. I've had two/three other careers and I think I know how a job should reward you. At the moment, I don't think flying cuts the mustard. I do hope things work out for you and your son and if he decides to go flying, I hope he works for a nice mob. But get the degree done first!