As long as there's always dry food out, any amount is probably fine. Presumably, if he eats less wet he'll eat more dry and vice versa. I have a can of Chicken Soup for the Kitten Lover's Soul laying around and it recommends 1-2 5.5 oz cans per day for a kitten under 4 months and under 5 lbs. So, if you want him on a diet of about 50% wet, start at 1/2-1 5.5 oz. can per day and adjust as needed. You can start by putting down 1/4 can twice a day and see how much he eats. If he doesn't finish it in ~30 minutes, you can cut back. If he gobbles it all down and wants more, you might want to go ahead and feed him more each time, especially if you want to try to get him to eat the highest % of wet food possible.