Your SO should've discussed it with you, how would he have felt if was the other way round and you didn't discuss it with him. He's selfish, he knew when he discussed it with you a couple of months ago that you weren't happy. Does he think money grows on trees? You need to take control, by that I mean, you need to take charge of his wages, pay the the important bills like mortgage/rent, gas/electric, phone etc. If you have any loans see if you can work out a smaller payment. Does he have credit cards? If he does then cut them up and cancel them, and work out how to pay any amount owed in smaller payments. Get all this down on paper on what your outgoings were each month and let him know how short you'll be with him cutting his hours. This is not your fault but, I feel you're the mature one that can take control.
Making your own lunches will cut costs when it comes to his work.
Let him see how much has to change now his wages are less.