Getting his weight after the morning bomb at least weekly is important so you know his normal weight. This is a key indicator should it start dropping all of a sudden. When you say he looks skinny, thats very subjective and really does not tell much. I would not reduce seeds to try and force eating more pellets on a bird. If he stopped eating the pellets and you were only providing a very small quantity of seeds, he could certainly be losing weight. The key is to try various high quality pellets and see if one is a hit he will eat some of. I never recommend adding vitamins to water. There is no way of knowing how much they are actually getting and it fouls the water. As long as you provide a variety of veggies, seeds, nuts etc. for a well rounded diet, pellets are not needed at all. Also, if you do not have avian lighting over each cage, you should for vitamin D3 production. The dark green poop just indicates he is eating some seeds. After the morning bomb, smaller frequent poops are the norm for all parrots.