It seems like there is so much he could do that doesn't require paying for new assets or more expensive artists.

For example lighting and shadows can be generated dynamically (currently they're baked into the assets, meaning they are inconsistent or completely missing). Or generate edges between ground textures procedurally so things don't look so blocky. Add a particle system for weather and atmospheric effects.

If he did that then he'd be able to carry that work forward into future games, and achieve a consistency of look and feel that extends beyond what he can get by trying to match artwork.