When the world is going by at 200 knots, that Cessna/DA-20 will feel like a go kart compared to a Corvette. It did for me!
A lot of what makes a good student pilot is being able to think ahead while flying. In that respect, doing some flying can help you get in the right mindset.
UPT is a demanding program, but a lot of it has to do with the flow of information. When you start T-6s, you get a paper box FULL of regulations and lesson handouts. Students normally cover half of that box during academics. Then, when they hit the flightline, it's aircraft limits, emergency procedures, local area procedures, maneuver parameters, VFR and IFR regulations, etc. For my first month of T-6s, I was studying or flying for 12 hour days, then going home and trying to study for another couple hours before going to bed. It's that pace, plus an attention to detail that gets people in a downward spiral.
For trying to get T-38s, students will need a good commander's ranking (usually with a good attitude and trying to help everyone else succeed), good daily ride scores (basically their "average" has to be good), very good checkride scores (biggest flying % of the ranking), and good academics. Checkrides and commander's ranking have the most impact, but daily rides and academic scores can be that extra bump that pushes people into the top spots. When I arrived at Laughlin, one student had missed exactly 1 question during academics and testing (something like 400+ questions on the tests). Sometimes it can come down to who did better on those tests in phase I.
BUT......
First things first! To get that chance he needs to get a commission and a pilot slot. IMO, the best way to do that is get into the academy and do average or better.