Feed him whatever he will eat, put the pellets with the seeds and slowly increase the pellets and decrease the seeds to get him off a high seed diet. Soaking the pellets in juice is fine to get him to eat them, he needs to get at least 50% of his diet in pellets. Maybe try different brands of pellets, most of us here like Harrison's High Potency pellets.
Offer different vegetables and fruits, more veggies than fruit, in different ways, cooked, raw, mashed, slivered, chopped or whatever form you want to offer. There is a thread where it has the ones you should not feed to him, make sure you read that to know what not to offer.