It's been really hard to tell when he's hungry vs having teething pain vs being tired, since he sucks his thumb for many reasons (that's the only hunger cue that I can identify). I would try to put him back on unless he'd just spat up a lot (he's been having spit-up issues since July; doctors haven't been able to find a cause). Then I would try to wait around an hour, because I haven't heard consistent things about feeding after spit-up. Some people have said to wait, some said don't. (This time the doctor said not to wait any more.)
Edit/update: I tried feeding him bottled milk after I breastfed this morning (2 hours after the previous feeding, which was "breastfast"), and he pretty much kept spitting out the nipple and not wanting to drink. Is that a good sign that he was getting enough milk, or could that be an indication that his stomach has shrank too much over the last month?