Going to be so much clickbait about this but strip it all back and only 3 viable options exist.
1: stay and don't extend. (he will be slightly cheaper to buy in 12 months time. But loses out on a year with a higher salary.
2: stay and extend with a clause which is lower than our currently asking price. This means he gets his bump with an out
3: leave this summer (the least likely imo as he's got no leverage).