I don't think you need to test too much today since you just changed the dose and things are in flux. BTW, slightly higher numbers are not uncommon just after a dose increase.
That's a nice drop - not too steep, but again, he is getting used to this insulin so he may still bounce a bit. That should even out in a few days. (paws crossed).
I have to run to the grocery store - hope to be back in an hour, but if not, I would go ahead and shoot 1.5u again. Unless he is a lot lower like 100 or something. In that case I would not feed, wait 30s mins and retest.