doesn't matter....a vet should be able to handle a hypo cat; that's something that you can't do at home if too low, ok? if the vet you know is nearby and open, it should be good enough.... that's if you need to go.
I am glad he is acting normal.... one of my civies normally tests around 42, but a diabetic getting insulin..... "Feed the 40s" and 39 is definitely a feed.
If you are feeding a food, give only tiny amounts as you don't want him to be full and refusing to eat.
Also, lots of cats get sick and vomit from too much karo after their numbers are back up, so don't be surprised if he's nauseous later.
Just keep testing every 15min and you want the numbers to rise, and stay up. You'll likely see a bounce later, so don't worry about that.
And 40 is up but not really a movement from the 39 and 37.... you want to see something above 50 and staying there, no drops or wobbles.