does tev need to exercise to maintain his muscle mass, or is there some scourge magic keeping him buff?

Nope, scourge magic made him a big buff cheeto puff, so he doesn’t need to exercise to maintain it. It basically froze him at the state he was in when he was originally raised, and nothing but the direct application of necromancy will permanently change that. (Everything else will heal over time, as he kills things.)

He still trains a lot anyway.