I have moved your thread to the Diet and Nutrition section.
The trick to get your Bubs to try out fresh greens is by continuing to offer them to him and to present the veggies in different ways in order to see what sparks his curiosity the most.
Some budgies prefer their veggies to be finely chopped up and placed on a bowl, while others love a big dripping wet leafy green to freshen up and eat.
Romaine lettuce is a good starter veggie and even the most stubborn of budgies will feel tempted and go for it.
In order to facilitate this process, you can sprinkle a few loose seeds over the fresh greens, this will likely entice Bubs to check them out and while picking at the seeds he can have a bite at the veggie.
Depending on the bond and on how attentive the budgie is to us, it's also possible to make them interested in trying out a new food when they see us eating (or pretending) to eat a certain food. This will generally work better on budgies and other pet birds which are more closely bonded to us.
You will find good tips and info on the stickies at the top of the Diet and Nutrition section.
Best of luck in improving your Bubs' diet!