Great to hear he's home, that's just wonderful! Oh there are plenty of things to put weight on an Amazon aside from pellets but some things you need to feed in moderate feedings when it comes to some fruits or some things that contain lots of carbs. I would start with some simple greens and move on up to sweet potato and just have fun experimenting on what he likes.
When I have more time tonight I will share with you a great holistic list of foods that you can offer all the time, some moderate and foods that should be given never ie; white potato