Congrats on your new rainbow lory!
We have had our chattering lory for 15 years. Her main diet is Nekton-Lori, which is a complete diet as far as all nutrients she needs. If you are also feeding a nectar power that is nutrient complete, then you don't need to worry about your rainbow refusing a fruit or veggie. Find something he likes... they're very fussy eaters.
They love the taste of the "juice" in the fruit or veggie, thawed frozen corn or peas is usually eaten by them, just the juice, they will leave the pulp. Grapes, for instance, cut in half, let him taste the juicy part of the grape. You just have to try a few different fruits or veggies to find what he will like. Eat it in front of him, exaggerating the pleasure while you're eating and that will encourage him to try it. You can also get a metal skewer and put half an apple on it... they usually love that. Again, they will usually spit out the pulp, while swallowing the juice from the fruit.
I will put the info for the Nekton-Lori nectar powder in my next post in case you'd like to check into it. I feed it dry in one cup with a cup of water right next to it. My lory dips her beak into the dry and then dips her beak into the water cup. She goes back and forth like this. It can get messy since they will shake their head to get excess globs of nectar off their beak, so I do it in the kitchen.