I think you are right, your ds is able to express much better than a pump. Your supply probably leveled off, too. Additional factor, your ds is having solids now, even though he didn't drop any feeds yet, he is probably taking slightly less milk because he gets some additional nutrition from solids. It is a natural weaning process, once you start offering something else than a breastmilk only, you will have less milk.
But, if you want to be able to express for his night feed, you could try either to pump in the morning, when your supply is the best and see if you have better results. You could also try to pump two times during a day, that way you would stimulate your breast to produce more. That probably would not produce results immediately but after some time. You could also try using herbs like Fenugreek and Blessed Thistle.
