2nd opinion on feline diabetes

Question
Does a serum glucose level of 300 necessarily indicate diabetes in cats?  I caught a cat I'd been feeding in my backyard, and brought him to a vet (a local cat shelter recommended him to me, as he deals with ferals and that's what my cat turned out to be) for shots/neuter, and this is what the vet told me (likely diabetic, should put him down).  But the cat was incredibly stressed, what with the trapping/car ride/vet visit and I thought maybe the high levels could be due to stress...?  Vet says he is between 3-5 y.o., and otherwise looks healthy.  I've been feeding/observing him for several months and as far as I can tell he looks like a perfect physical specimen of cat (muscular, healthy, not over or underweight, even though I provide him with a bottomless food bowl). Given the cat is so stressed, and the vet is reluctant to handle him again, I don't want to bring him in for the follow-up testing unless absolutely necessary.  If it is most likely diabetes, is it cruel to let him take his chances with hyperglycemic shock if I set him free, or is it better to put him down?  He will not tame without several months of hard work and there's no way I can grab him for daily insulin shots, even caged.  And he is miserable being caged.  Thank you.

Answer
It is probably most likely due to stress.  I would recommend a urine sample be checked at the same time as blood sample to see if there is sugar in the urine.  Also, you could have a fructosamine level checked to rule out stress factors...the level should be VERY LOW if no diabetes.  If he is truly diabetic, then it is not fair to let him outside without treatment.   You could try feeding Purina DM or Hill's M/D, which is a high protein diet designed for cats with diabetes.