adding or changing to their diet at this age is a bit difficult. give him a variety of veggies, you will just have to experiement with what he likes, and what he doesnt. my bearides are 3-5 years old, and over the two years ive had them i have actually gotton them to eat more things than what they did when i first got them, as they would never touch it before, now they love squash. if you want to try something new, just put it in a dish in their cage, and leave it for a bit. sometimes if i mist thier food, they take more interest. just be patient, it may take a while
good luck with your new friend
