My beardie is roughly 4-6 months of age and has grown to 11.5 inches in length from tail to head and I remembered someone told me that when they are around 6 months old they are considered juvenile and require vegetables in their diet. Now my beardie roughly eats 60 small dubia roaches a day so he has a large appetite. I just wouldnt know how much vegetables I should substitute for roughly 20% I think it was at juvenile stage.
Offer a small amount of veggies daily, honestly, you could have started when you first got him...
Some will eat veggies regularly at a young age, others may snack on them and then with some it's a constant battle when they are younger.
Dont be alarmed if he isnt eating his veggies, he will eventually just give it time. With one of my beardies it was a struggle to get her to even realize veggies were food until she was a little over a year old.