The initial signs for diabetes in an already overweight cat are, typically, weight loss (though not always) and increased drinking and urination (PU - polyuria - excess peeing and PD, polydipsia - increased drinking). The PU/PD can be indicative of other ailments (kidney issues and hyperthyroidism) and these can be determined on regular blood panel at your annual vet visit.
There are other signs - like I saw with my cat - when the diabetes has been around for a while - sometimes weakness in the hindquarters (diabetic neuropathy). Let's hope Philippe never gets to that stage!
I know Sue gave you a site to look at - definitely go to catinfo.org to see how to safely manage his weight loss.
Yes, Type II diabetes is preventable in cats, just like humans! By the way, cats also have the wonderful ability to go into remission (i.e. be diet controlled and completely off insulin).
Great that you are here.