I started throwing just FH. Teerex is a prettty extreme disc to start with. Def think less overstable and/or slower would be the way to go. However, since he actually likes the Dx Teerex, neutral discs like teebird would probably just flip over on him in a heartbeat, so still need some speed/overstability to crank on with that sidearm snap.
Just as a point of reference, I drive FH with a 170g Pro Wraith. Not quite a teerex, but similar speed at least.
With all that in mind, here are some other good discs to try.
DX Viking- flys straight with a small, predictable fade at the end. Less S than a wraith. Champion version fades much harder at end of flight. This disc does not like to S curve.
Viper-still overstable, but slower. The more you beat it in, the straighter it will fly
Or perhaps an Orc or starfire
Keep in mind hes used to a dsic that has no High speed turnover, and significant amt of low speed fade, so the key should be replicating that in a lower speeds/lower fades, which should lead to a striaght flight. Many discs mentioned, like the Beast & Valkyrie all tend to S-curve due to turnover, which is not what he likes about the teerex, im guessing, so switching to a beast would be a totally different animal, methinks