2 problems,Vinnie and Chico

[chico2]

First off,Vinnie(almost 9 yrs old) is getting obeseif I did not know better,I'd think he's depressed after Rocky passed,he was his pal..
I know he needs exercise and I try every day to play with him,featherwand,balls whatever he used to like.
He used to like to come outside even in the winter,but now he comes out for 2 seconds and that's it.
When Rocky was alive,he kind of led both Vinnie and Chico outside and they'd run around.
All Vinnie wants to do is,eat,sleep,cuddle and get brushed...how on earth can I get him to lose weight.???

He eats one half 5,5oz can of Wellness or Natural Balance for breakfast,the same at dinner.
Lunchtime,either a few pieces of cooked chicken,or some pieces of Blue Buffalo dry for a treat.

My vet is not happy with him,he might end up diabetic,just like Peppi another white cat I had.
So far,the vet has not suggested any of the prescription diet food,which I really do not want to give Vinnie...but what do I do..
He has to lose weight.
Raw he will not eat...any help appreciated...thank's !