Colour-changing is very normal in bearded dragons. A beardie's stomach is naturally white, and the black patterns are commonly called "stress marks" because they can come in times of stress. They can be a sign that your lizard is hungry, too cold, too hot, etc. Many beardies get them when they're shedding or basking. Younger dragons tend to have them really often. I wouldn't worry about it. If you're tank is decently sized (30 gal for hatchlings/juveniles, 50 gal for adults), has the correct temps (85* cool side, 100-110* basking), is equipped with the proper lighting, and your beardie is well-fed/
hydrated, then I wouldn't worry about it.
He may get more red because he's happier outside of his tank getting attention from his humans. n____n the white belly is also a sign of that. He may just get really happy when eating. I'm not sure, but it is popularly believed that beardies get darker when basking because they are absorbing heat.
Bottom-line: Colour-changing is normal. Don't worry about it.