Certainly exercise will lead to weight loss, but if the weight loss is sudden and activity has been constant you should have your pet checked out.
For a dog like this at the D.E.L.T.A. Rescue Sanctuary we start with a good physical exam with a fecal exam. Then a screening blood test and survey x-rays. These tests should detect any health problems. If all these tests are normal a change in diet may be required to provide enough calories for the activity level. We do have a few dogs at the sanctuary that get canned food three times a day in addition to their dry food! Large dogs like this should not eat a large meal of dry food and then exercise right away. This can lead to bloat.
You imply in your question that food is down for nibbling all the time. this is ok since it generally prevents engorgement on the part of the dog. If meal feeding, it is better to feed smaller meals, more often, and prevent exercise for an hour after eating.
