Tim:

Sounds like you might have been trying to fly it too fast. The Nexstar is designed to fly slowly, with an OS 40LA. If you were using an ST 51 and a 10 inch prop, you should have been flying at half throttle. The faster you try to fly the Nexstar, the squirrelier it will get.

That wouldn't account for the "cartwheel" on take-off, though. That sounds like tailheaviness or too much elevator travel. I know you said you rechecked those things, but could you be reading the recommendations wrong?

All things considered, your brother might want to try an Aerostar (Midwest). They have a somewhat broader useable speed range.

Jim