I learned from my vet, that feeding only canned food is like us eating fast food every day. Perhaps you could give him 1/4 to 1/2 can per day, with a nice dry food to help him slim down and get healthier. I feed my cats multiple times per day, but only a little bit at a time because mine tend to "scarf-n-barf" if they are fed all at once.
I sould suggest feeding at least two times a day, once in the morning and once in the early evening, around your dinner time.

As far as spraying is concerned, there could be several reasons, from not being fixed, to being overweight, to having a behavioral problem or health problem.

Let us know how he's doing!
**hugs**