Periodot does look somewhat underweight in the picture. My budgie, Shelby, who looked like Periodot, was underweight as well which is why I say that.
Have you tried mixing very finely chopped veggies with some hard boiled egg?
You can also try sprinkling a few kernels of sweet corn with garlic powder (not salt!) to see if that will entice him to try them.
Has Periodot been seen by an Avian Vet yet for a well-birdie checkup?
If he won't eat anything else, then increase his seed by 1/2 teaspoon per day to at least get some nutrients into him.
Have you tried sprouting the seeds - that would be more nutritious and he might like them that way!