Welcome Danamarie, sorry to hear about the loss but congrats on the new fid in your life. Its always a good idea to get to an avian vet or a vet experienced with parrots after bringing home a new bird even if he was 'vet-checked' or even under the care of a vet at the breeder or store.
The weight issue is certainly one a vet could address, particularly whether his current condition is underweight.
That being said, if he was not on the best diet where you got him from it can take some time to gain it back. Also consider that he is young and growing, it may take him time to fill out.
I know this was all fairly vague; only a vet who has checked him out could give any good specifics.
As far as foods to "fatten" him up a bit there are some. First off though is a good base diet, which I suspect you already are familiar with since you've had birds: a quality pelleted food is a staple which should be supplemented with fresh fruits and veggies. Beyond that boiled eggs or chicken (baked, boiled, or cooked in minimal oil) are good nutrient dense and healthy supplements to help him put on weight.
The weight issue is certainly one a vet could address, particularly whether his current condition is underweight.
That being said, if he was not on the best diet where you got him from it can take some time to gain it back. Also consider that he is young and growing, it may take him time to fill out.
I know this was all fairly vague; only a vet who has checked him out could give any good specifics.
As far as foods to "fatten" him up a bit there are some. First off though is a good base diet, which I suspect you already are familiar with since you've had birds: a quality pelleted food is a staple which should be supplemented with fresh fruits and veggies. Beyond that boiled eggs or chicken (baked, boiled, or cooked in minimal oil) are good nutrient dense and healthy supplements to help him put on weight.