As I stated in your last post, for the most part it's not quantity, but quality. If you are intent on switching feeds, you should do so gradually (most of us switch over a one month period or longer) to avoid stomach upset and diahrea.
A one month switch would look something like this:
Wk1: 1/4 new food to 3/4 old food
Wk2: 1/2 new food to 1/2 old food
Wk3: 3/4 new food to 1/4 old food
Wk4: primarily all new food with just a bit of the old sprinkled in
As for amount to feed, I would probably go with 3/4 cup per feeding twice a day with an afternoon and evening snack. You can mix in a tsp of yogurt (plain, lowfat) or cottage cheese (lowfat) to help him gain weight. Also, if you haven't already gotten him one, pick up a kong suitable for his size (don't use the kong treats, they are a pain in the butt to get out of a kong) and fill it with a mix of yogurt, fruit, and peanut butter. You can freeze this mixture or not. The kong will give him an alternate means to exert some energy and exercise his brain and get a healthy snack too. You could also give a peanut butter sandwiche for a snack. We have some great Boxer Belly recipes in the Boxer Belly section of the forum.
My Boxer eats Merrick brand dog food. It's very high grade, natural, with no preservatives. She has increased coat shine, has put on a bit of weight (healthy amount), and even has softer paw pads.
Please keep in mind that unless your vet tells you that your pup is underweight, you shouldn't really focus on "beefing" him up. Boxer are naturally slim dogs. They are genetically built that way.
Also, if a dog is stressed they will be less interested in food. And if you switch feeds to often the dog can become finicky and picky. So switching and then sticking, at least for a bit, is the best route to go. He may also not be eating much because he just flat out doesn't like the feed you're giving him. Sometimes finding the right feed is trial and error.
Best of luck and hope this helps...