Junior assigned to a 10 year old project

I find it strange that he says he's not learning anything. It sounds like he's in the perfect position learn everything he needs to embark on a nice long career in corporate programming.

More seriously, refactoring a neglected codebase and bringing up to speed with modern tools and best practices is probably one of the useful real world skills a professional programmer can have

I feel your pain. But, really, that sounds like a typical junior starting point, at least to me. It's a major pain but most places aren't going to put a newly hired junior on a major project his/her first time at bat.

Consider it a learning experience, and make yourself available when/if other projects are mentioned. If nothing else, consider it training for a future position. Quitting right away will only look bad down the road to future employers.