Agree that If he wants to be a DI coach first and foremost, the shorter route is still probably to stay at Mason/DI as an assistant.
If he goes to DeMatha it's because he wants to coach DeMatha long term. There are a lot of reasons to do so--more job security, more media coverage than a ton of DI head coaches get, coaching some of the best young talent in the country, etc.
And if he goes to DeMatha it certainly doesn't disqualify him from being a DI head coach down the road. He already has enough college assistant experience on his resume...