Actually, cockatiels don't eat fruit at all naturally (I live in Aussie where they come from). It is extremely rare for any tiel to accept any fruit as not their natural diet but I've heard about the occasional one that has a penchant for a particular fruit. Avoid the fruit for a tiel, waste of fruit
I feed my tiels mixed frozen vegies soaked for several minutes in boiling water after making my cuppa to par cook and soften. They love, love, love mashed potatoes, or cooked potatoes of almost any kind, even chips, any pasta, noodles, rice (sometimes but not always rice). Any meat they love, especially chicken (cannibals). It does differ from bird to bird but you just have to persevere. All of my tame tiels over 20 years love chicken and will fight you for it. He'll get it. He might look at the colour like a play toy, ahh...but then he gets the taste! Just keep offering day after day. My tiels LOVE corn, peas, carrot not so much, broccoli maybe for some, cauliflower, beans at a stretch or not, greens...they SO need greens but not lettuce as devoid of anything good for all birds.
They are smart little birds and have the most amazing memory for what they like to eat. Most of my tames would watch, look at what I am cooking. They learn to recognize what they don't like and pester me to death for what they do like.
On seeds and pellets, tiels actually eat seed in the wild but not as a total diet. Pellets are fine as they are nutrient enriched to help their needs. Keep persevering with veggies and greens. Let him see what you are preparing and he will have a whole new interest, as in if parrent eats it, then okay with me.
Nothing from the onion, mushroom, tomato family as can kill them. You probably know that avocado will kill them if eaten. Hope this is of some help.
They will chew what the choose to chew around house. All you can do is steer them to something that is okay. I tell my tame tiels over the years a sharp no. They sometimes learn.