He went about as far as you can, in terms of rank, without becoming an officer. Generally, you can't transition from enlisted to officer (become a "Mustang") after about age 28 or so.
Warrant officers are "super sergeants". There are four warrant officer grades W1-W4, so there's room for advancement within the warrant ranks. Usually, they are former enlisted men with special technical skills.
When I was in ('74-76) we had a forty year WO who had been around for the Big One. Had to be a different perspective for him.
There are five Warrant grades now. CW5 is usually called a Master Warrant or MW5. Those are usually the real experts and some of the older warrants. We have several who serve as technical advisors in the avation and maintenance fields. There are also a few MW5 in the mess halls.