Hello, and welcome to the Parrot Forums family!
The advice from the above two posts is excellent. You should definitely get a scale if you don't already have one. Like Wrench, I weigh my birds daily. Very helpful tool since they have such fast metabolisms.
One other thing to add, you can check for yourself if your bird is underweight or not. Feel for his keel bone. The pectoral muscles to either side of it should feel firm and nearly flush with the keel bone. If the bone feels sharp and prominent in relation to the pectoral muscles, then he is likely underweight. (And if you have trouble touching the keel bone because the pectorals to either side have grown beyond it, he's overweight. Not your issue, I know. But just so you know.)
And lastly, if it does turn out that he's underweight, you should take him in to a certified avian vet just to rule out anything else going on.
And as the others have said, it'll help if you give some idea of your bird's current diet. Otherwise, it'll be hard for anyone to give suggestions as to what you might add.