There's the problem; you can't build decent lather in a cup - x10 if you're trying to make lather in the same cup the soap is already in. You need room to whip it around. Load the brush on the soap, in fact, overload it. Then move to a bowl or your face, and add water slowly.
+1
I think you have found the root cause of his issues. You need an adequate sized container to allow the brush room to work. A cup sized vessel is 100% too small. And lathering on the soap is an absolutely horrible way of making lather in my opinion.
-Locate a bowl that is fairly deep, and has gently slopping sides. A rice bowl, or large cereal bowl should work fine.
-Shake the brush out 3-4 times, and give it a good squeeze. You want to start out with a fairly dry brush.
-Load the brush on the soap for at least 1 minute.
-Take the loaded brush to your bowl, and begin to generate lather. Add water in small amounts 3-4 times, while you build the lather.
-Good lather should take about a minute to two to generate. It should have the consistency of yogurt, not cool whip. (cool whip like lather is way too dry)