Fake flying is common in caged birds. I think they just get fidgety and are stretching and burning off a bit of excess energy. (Mine usually go airborne and hover when they do it, but they're older and probably stronger of wing than Gun is yet.)
Once you have him trained and can safely let him out to fly around a bit, he'll probably do less of the fake flying.
I don't like the sound of the watery poops for more than a day, since it sounds like he's eating mostly pellets and seed. But if it started at the same time he started actually eating some of the spinach, it could be from that. Or if it started when you cut back on seed and offered mostly pellets. Diet change often causes a minor, temporary upset.
But if it's not from the diet change, then yes, you're right to worry. If he's still got watery poops today, a third day, you should try to get him to a vet today. If he's actually sick, Monday could be too long to wait. That would be the sixth day of illness (if he is ill).
You're giving him plenty of fresh, clean water, right?
Oh, and if you're putting vitamin supplements in the water, maybe you should stop, and clean the water dish thoroughly. Those supplements can promote bacteria growth in the water, and a simple once-a-day changing of the water isn't good enough to keep it clean.
You aaked if he's eating enough, but you didn't say how much he's eating, so I don't know. If it's not at least a tablespoon total per day, that's not enough. Mine each usually eat two tablespoons of food in a day, but occasionally eat as little as one or as much as three. Does that help?