This is not that difficult:
1) His current contract contains the vesting option that kicks in if he gets those 1050 Plate Appearances over the 2017 and 2018 season. As of today, he has 748.
2) If another team claims him off waivers or makes a trade for him, they assume the terms of his current contract. This makes it extremely unlikely that any club will take him in this manner.
3) Once released (in a week), he is free to sign a new contract with any team. Any plate appearances from here forward will be under that new contract, not his current Red Sox one. He got 748 PAs under his current contract; that is not 1050 and the option does not vest.