How do you get a budgie to lose weight?

My vet said to weigh seed every day and only give him 5-10 gms a day. It worked initially and he went from 60 gms to 40 gms in three weeks, but he quickly put weight on again and now weighs 50 gms. I've been following my vet's instructions, but it's not working as well and I'm beginning to think something may be wrong with him. He is much more energetic when he weighs less. He seems ravenous, but only eats a little of his spinach and other fruit and vegetables, even if I take the seed container out of the cage during the day. The vet said I should get him onto dried pellets or crumbles for birds, but he doesn't eat much of that either, really prefers seed.