Nancy, as the others have said, since you have no history of his weight, perhaps he is just a small, not average size bird. I know Talon is on the small size, but don't worry cause she has always been smaller. My vet says she's very healthy, just tinier than average, but that is normal for her. You may be upset for no reason. If he seems happy and thevet says healthy, the you can relax.
I would definitely weigh your birds on a regular basis. I weigh mine monthly, write in on a sticky note, date it and add to it each month to compare. I keep it in them drawer right near my scale.
Seems you are doing all the right things, as you would say, trust your instincts.....
Also, try some oatmeal, my birds love sharing mine with me every morning, being fed by spoon of course!!