I would not stay at the same dose for ten days. You can raise the dose after three cycles. Most people find it easiest to raise at four cycles so that they are always increasing on a morning cycle since it's easier to get mid-cycle tests during the day.
If you look at other people's spreadsheets, you'll see that most of us do a test before each shot, and try to get at least one mid-cycle test each cycle. So I usually do a total of between 3-5 tests each day. It's the mid-cycle tests that tell you if the dose is working, and help to know if the problem is rebound as your vet suggests, or not enough insulin. Based on the mid-cycle test you've done the past few days, it does not look like rebound to me. It looks like OJ just needs more insulin.
Many vets, and it seems this is particularly true in Europe, don't think a cat should need more than 2u and they get nervous at higher doses. Unfortunately, many cats need more than that. If you are more comfortable taking your vets advice and want to lower the dose, you can do that, but I would strongly recommend if you choose to do that, that you be prepared to quickly move him back up through the dose changes and don't leave him down there for very long.
Instead, I would suggest that you start working through some increases every few cycles until we start to see some better numbers. If you tend to run out of test strips, buy a couple of extra boxes to keep as back-up in the cupboard. Especially if you're going to do the increases more consistently, you'll want to make sure you're prepared.
