It is healthier for parrots in captivity not to eat seeds. Seeds are high in fat and low in vitamins - like children with sugar, parrots will always choose seed over pellets.
You should ween your Green Cheek off his fatty seeds and onto an all-pellet, fresh vegetables and fruit diet. This can take a few weeks. Firstly (if it is not done already) you will have to mix his seed in with his pellets so he has to rummage through the pellets to get to his seeds. Over a few days, slowly decrease the amount of seeds that are mixed in with the pellets. He will start to realize that pellets aren't so bad when they make up most of his daily food allowance. You can still use seeds as treats or as rewards for good behaviour.
Ultimately, you want your parrots to be eating around 65% pellets, 20% vegetables, 10% fruit and 5% nuts and seeds (obviously this is just a guide - the more fresh foods the better!).
Good luck
