Without being able to see the same numbers you are seeing I can't even guess. It could be one of two things...too much insulin or too little insulin but without being able to see the patterns I don't know. But if his numbers keep getting worse as you increase the dose then the only way to go is down in dose and see if he gives you better numbers.
There could be a myrid of reasons for him to constantly rise all day...Since he goes outside, he could be getting into something high carb, it could be not enough insulin, it could be too much insulin. He could have an underlying health problem which could be something as simple as an infection or a complex as IAA. (insulin resistance).
As much as I wish this was a simple equation of if the cat weighs so much, eats this and has a BG of this then you give this much insulin, it just isn't that easy. The other thing that comes to mind is how old is your insulin?
Mel, Maxwell, Autumn & The Fur Gang