Constipation after treating Diabetes?

Is this something that can happen after treating diabetes? For the longest time he had normal stools, and the occasional very soft stool or diarrhea. After 2 weeks on 1U on Lantus, he now has very hard stools, and occasionally he can't get it out in the litter box, and will walk around dragging his butt until it passes. He hasn't eaten dry food since last September, and he only eats boiled chicken and canned wet food in gravy.

Is this a common issue? How can I help him? It doesn't seem very comfortable for him.

 

Any recent food changes? Appetite or water intake changes?

https://felineconstipation.org/acute-treatment.html

 

None, the only change is we are now treating Diabetes. Because he is both a CKD and now a Diabetic, he does drink a LOT of water, but he also gets water in his wet food. He eats more or less the same amount, and the same food, day in and day out. He doesn't like change.

 

I should mention that he is passing these stools on his own, just with extra work and it doesn't seem comfortable.