Welcome! Sounds like there are a lot of things going on here.
Yeah I never get anything at all if I try pumping after nursing, even when she's only had a few ounces.... I HAVE to pump when my breast is full or it just doesn't work!
We've had many times over the past 11 months when she seems to be really angry that there's not enough milk. Sometimes I've attributed it to a growth spurt, sometimes she's just cranky and doesn't want to nurse (doesn't sound like your problem). It could just be that he's spurting and wanting more than usual.
I've heard from several friends that power pumping works great to get supply up. Also, have you tried nursing him more frequently? The way the breast fills is very quickly when it is empty and slower as it gets fuller. So by feeding more frequently the breast fills up more quickly and though they'll get less at one time they'll be getting more milk overall.
I've never supplemented, even with frozen breastmilk, because I'm very leery of my body thinking it doesn't need to produce that extra milk... the only way to really get it up to the supply the baby needs is to have the baby keep demanding milk so that production increases.
That sounds like an okay number of wet diapers to me.
Overall it sounds pretty normal to me... and it doesn't mean your supply is dropping or that you won't be able to continue nursing. Hang in there! Most phases last about a week or so, in my experience, and then things get back to smooth sailing. I hope the power pumping and nursing sabbatical help!