Not being able to defecate is a problem. He may be obstructed or have another problem. I'd suggest any vet is better than no vet at this point.
You'll want a vet who:
1) supports home testing of blood glucose and is OK about using a human glucometer
2) supports feeding a low carbohydrate, over-the-counter canned or raw, food
3) is either up to date, or willing to do the homework to become up to date on feline diabetes and common comorbidities.
4) if up to date, is familiar with the use of Lantus, Levemir, ProZinc, and/or PZI, IN CATS; ask how they determine initial dosing and how often the dose is given (should either be based on body weight or starting low at 0.5 units and dosing should be twice a day at 12 hour intervals). Also, does NOT prescribe Humulin N, which lasts only 8 hours in the cat.
5) has pricing you can live with - check office visit fee, vaccine fee, CBC and chemistry fee, etc. Ask how often they have you bring the diabetic cat in and for what reasons (if you are home testing, you can do curves and averages at home, and the cat doesn't get 'regulated' at the vet office where stress makes the numbers unreliable)
6) if currently treating diabetic cats, ask how long they usually survive (should be several years)
I'm an hour away, in Central Columbus. I'm free some of this weekend, if you need me.
