An MPL will become an ATPL upon 1500 hours and completion of the relevant skills test, in exactly the same way that a "frozen ATPL" becomes an ATPL.
I'm led to believe the only risk with an MPL is that your licence isn't much use for another type. So for example if you lost your job with Qatar on the A320 you may run into issues finding work on another aircraft elsewhere. I could be wrong on this though as I completed my training whilst the MPL was still in development.
My gut feeling would be that it's a great opportunity and the company aren't going to be disappearing anytime soon, realistically most people wouldn't move on from their first jet job until they had 1500 hours anyway.
With regards CTC v Oxford etc, there's plenty written on these forums but my perspective is they are much of a muchness. I trained at one of them and I would probably be in exactly the same position had I chosen either of the other two. The main consideration is how good they are at helping you find work once you complete the training. At the end of the day, you leave with the same piece of paperwork and however much work you put in trying to find a flying job, they are still the best placed to help you. Considering your son would be entering a mentored scheme, that takes away some of that decision making.
Good luck to your son!