^ Second what has been said above.
And I use this website to help me calculate the calorie amounts in food to ensure I'm feeding my cats enough. (or not too much, as food labels will often try to get you to feed more than they actually need, and/or are vague in exact amounts)
Cat Food Nutrition Calculator | Elizabeth C Scheyder
Most canned foods will also list how many calories there are per can. (if you can't view the cans yourself, then the manufacturer's website is your 2nd best bet) I used to make the mistake of going by eye, assuming that a 5.5oz can was plenty for any cat. But all foods have VERY different calorie amounts, so it's good that you already plan to check those amounts beforehand.
As for figuring out how much your cat needs.. well, it's easier said than done. lol. Starting out with his ideal weight is a good point, then factor in activity level, whether he is indoor/outdoor, etc.
The recommended amount varies heavily depending on these factors, but is around 20-30 calories per pound. 25 is usually a good starting point, as 20 is pretty low and 30 would probably be for super young, active cats. But again, it isn't an exact science.
I myself am STILL working on figuring out a more exact calorie amount for my overweight senior. In my experience, they shed weight very gradually, so you want to avoid accidentally starving your cat just because they aren't losing as much weight as they're supposed to. I feed Scamper enough food for a lean, healthy cat, yet she still retains a fair bit of weight. (though, she is also a bit old, indoor-only, and very inactive)
Once you start mapping your calorie plan out, be sure to post if you need any brand suggestions. There are lots of us here with great recommendations.
Best of luck to you and Walter on his weight-loss journey.