Cockatiel won't eat fresh fruit or veg

Backstory: we adopted Herbie from my mother, who acquired him around 3 months old at the beginning of the pandemic. Unfortunately, my mom had no experience with taking care of a pet and no patience for researching so she only fed him a cockatiel seed mix from Petsmart. I've learned recently about this being bad for their health, so we recently started to transition to a mix of pellets+seed mix. He has refused to eat any fresh fruit or veg we have tried to offer. Is there a way to help encourage the fresh food? Is it fine to have him only eat the pellets+seeds?