Welcome!
Possibly because he likes sweet things. My Emma's conure doesn't eat seeds either, perhaps millet as a treat. Making him eat vegetables did prove very hard.
I suggest try going for tasty and sweet veggies, starting with red bell peppers if you can get any. Try leaving it in a bowl with nothing else, and when he is hungry, he should at least give it a try. Peppers have a sweet taste, and mine seems to love it. Other type of peppers are a no go for him, so I try to give him the red and yellow peppers at the least. they are quite healthy and do not contain so much sugar compared to fruit. Another thing my conure likes is the middle of the carrot. Yeah, just the center, i kid you not.
If he likes fruits, I highly suggest them pommengranate. They are super healthy and packed with vitamins, so you can give some of those.
Keep experimenting with other types of food, through time he should become more acceptive of other types of food.
Is your plum head truly eating ONLY fruits or is he eating anything else besides it? (millet, other seed mix, pellets, etc)