Well they won't regrow unless he pulled them out including the follicle. Otherwise you have to wait for a molt.
For mine, I give them the same thing I do for my regular birds: 90% or more of a complete feed, 10% treats or less. For molt time I like boiled egg yolk mashed, yogurt weekly. Fresh clean water.
If I can't smell my feed and smell its freshness, if the feed smells like cardboard, I get a new feed supplier and supplement with vitamins in the mean time.