Try some different veggies. I havent had my Dexter long (3 weeks) but I do find that he favours some veggies over others, squash is his favourite, he always eats it.. this week we tried bok choy and sweet peppers and he turned his nose up at them. You may just have to find what he likes.
Edit to add: Try moving the veggies around with your finger in the bowl, or wiggling them in front of his nose... Beardies prefer food that moves.