His weight sounds OK. Boxer puppies tend to come in two "types" - the ones who grow upward very quickly and are very ribby as a result (they're often very energetic pups, but all the food they consume goes into growing taller and in outright energy needs); and the second 'type' are those that grow in a more equitable way (meaning upward and outward at the same rate). Those pups get taller more slowly (of course - they're growing outward as well as upward), are typically heavier, less energetic and sleep a lot. Both are perfectly normal boxer puppies

and it sounds as though your boy is in the first category. In that case, he'll probably be ribby until at least 18 months or so, when he's finished growing upward and starts to put his energy into filling out
If you do a search of previous threads here, you'll find literally hundreds of similar queries from people who've been concerned about their puppy's weight and being able to see their ribs. But it's normal
Puppies also tend to grow in spurts, and their nutritional needs vary accordingly. So you might find you go through periods where your pups is constantly hungry, and others where he just isn't interested in food. If you think your boy is on the thin side and he WILL eat more food, then (within reason) it makes sense to give him more when his body needs it. Just don't worry if sometimes he isn't interested.