Sounds OK to me. But whether or not they're under weight depends on how tall they are (30lb at 17" tall would be fat, while 30lb at 21" is probably a little thin).
Regardless, boxer puppies very frequently are ribby. They put most of their energy into growing upward, and the filling out doesn't commence until all the upward growth is complete. That means that a great many pups are ribby until at least 18 months of age - and not completely filled out until they're close to 3 years old. There's nothing you can do about it either. If you feed them more, they simply grow taller faster, and that's not good for them. And they still don't fill out...
Looking at your pup's pictures in your gallery, I'd say that is the case with both of yours. Yes, they're on the thin side. But not unusually so for boxer pups of their age. As long as you're feeding them a good quality food, and their appetites and activity levels are good, then there is no need to worry about them. They will fill out in their own time, but it won't be for quite a while yet.