I'm definitely concerned about the lethargy and staying near the water dish. Is he eating ok? I would make sure of that 1st. If not and if he seems to need the dry to feel better, I would let him have dry food and then it'll just be more of a challenge most likely to get the dose right. But eating is the most important.
If it were me, I think I would try 3u (or 2.75 as Sue suggested) for 2 full days and then reevaluate, checking at least once per day (and more if it's an easy feat) for ketones. Because that last 2.5u was 2 hours later than a normal shot, that is why you didn't see better results on that cycle. The fact that he didn't rise all that much in my experience has been an indicator that if you shot that same dose on a +12, you would get some drop. I agree that since then it's hard to tell what is resistance to the dose.
Typically you want to hold a dose for at least 3-4 cycles as they can often get a better response on cycle 3 or 4 of a new dose. Although you can change PZI doses with every shot, normally you don't really want to do that when you are on a dose increase path. It's hard to imagine until you have seen it, but sometimes when you have the right dose you will get a really nice breakthrough to perfect #s. PZI seems to have sort of a sweet spot with the dose - too low and not much action, too high and you get weird #s, which could be what you are seeing.