He has plenty to lose. By not playing to the terms of the contract he agreed to, the team owning his contract would put him on the Restricted List, and he would be unable to sign another contract. He couldn't just "wait out a year" and become a free agent, he owes his team a year under those terms and they have the leverage to force him to do so if he wants to play baseball again.
Moreover, if he did play for that year but played shittily and/or grumbled to the press at every opportunity, that would be reflected in his next FA deal. He's on contract year status and has all of the pressure that comes with that.
Pretty much his only leverage is to retire. He can grumble about it, but major leaguers are far too competitive to just sit out rather than playing the game. It would be a truly rare set of circumstances that would cause a major leaguer to refuse to honor a contract - you just don't hear about players doing that here, as opposed to the NFL, because all non-option years are fully guaranteed.