Hi! I have two cockatiels myself and have ahd many in the past so I know how yuo feel with diet!
You're right, all seed diets are vey bad for birds. There's no set diet for parrots but more a diet per bird like cockatiel diet is very different than a senegal for instance. There are many options you can take for your cockatiel.
Firstly, cockatiels are not big fruit eaters but more prefer vegetables such as carrots, leafy greens, sweet potato, bell peppers and a particular favourite of mine chillis! The hot taste is ppealing to them as they don't sense the spiciness as much. Stronger tasting or certain tastes of foods are what's more important to birds in general but particularly cockatiels. Another resaon for them being scared of fruits is the bright colours, they see coours wayyy brighter than we do so a mango can be quite alarming! Also trexture and size is important, try serving the new foods in different ways; mashed, warm, chopped, squared, etc, you'd be surprised how often this works. Also cockatiels are natural ground foragers so perhaps consider buying a platform or feeding tray which you can use for new foods to encourage this instinct or alternative buy a birdy kebab a kind of stainless steel spear with NO sharp which you can hang from the top of the cage with various fresh items on. You can also use millet clips for large clumps of fresh leafy greens, mine love kale, sometimes spinach (this is not fed often), brocolli, watercress, lambs lettuce....
With regard to your pellet choice, I would rethink that personally as Zupreem are full of unnatural colours and ingredients. If you are going to use pellets, I would suggest using TOPS. They are organic, cold pressed (not baked or extruded) and use no articial colours or synthetic vitamins. However, a cockatiel any any small parakeets diet should be varied and while pellets do play a part and in my OPINION should be a part of it you do need to supplement with regular vegetables, fruits if possible, rice (brown and well cooked) or wholewheat pastas, homemade eggfood during moulting (birds need more protein during this time), pulses and sprouting mix.
Cockatiels should still have some seed daily (just a small portion)but you can make this healthier in a number of ways. I buy tidymix parakeet diet which excludes sunflower seeds and has special dried fruits and various healthier things, while it should not be a WHOLE diet its still a healthier seed portion. You can also sprout your seeds, the sprouts are very good for them and also help the seeds to become a healthier portion of the diet.
The alternative is to feed a completely raw diet, which consists purely of fruits, veggies, some seeds, pulses, sprouts, rice, etc. This is a healthy diet also although should only be undertaken with a great amount of research. I personally don't feel I have enough experience to implement this yet. There is a lovely poster called Parajita on the forum who could help you more with this. We differ in our opinions but if you DO want to go down this line I'm sure she can help you.
I use the following diet; TOPS pellets, 2-3 portions of fruit, veg, rice and pulses daily, small portion of seed which also contains dried fruits and veggies which they do eat. A varied diet I find works very well for my cockatiels combined with regular outside cage time up to 4 hours per day AT LEAST. This has worked well for me. If you are feeding pellets, make sure to feed plenty of moist foods.
The easiest way to introduce new foods other than what I have suggested is to eat it yourself on a plate but try to preent him from getting near it... after a while of him trying to eat it offer some. you'll be amazed how well this one works.
Change textures, shape and way of serving as mentioned previously
Change where you serve, cockatiels prefer to be fed on platforms where they can forage through the foods on offer rather than being fed in a bowl.
Try to hand feed, be patient and wait for him to come to the vegetable. Withdraw all food 2-3 hours before so your bird is hungry and see if he takes it. May result in you having an achey arm. If he's tame as you've pointed out this is a great activity for you both.
You need to have a great amount of patience for new food introduction, just keep trying, if he doesn't eat it don't give up on that thing keep trying and see. You will get there! Cockatiels are notouriously stubborn and fussy but you have him young so he will get there.
As for proportions of what should be fed I use 25% between pulses and rice type things, pellets, seeds and fresh foods but you can vary this depending on what your tiel can handle. At certain times you will find they eat more of one thing than another and a week later more of pellets than another. It depends on what they feel they need. It also might be an idea to offer the seed portion as a meal in the evening so he only has the option of healthier foods in the day. Please watch the birds weight though because you don't want him to get starved but at the same time you want him to be hungry enough to get new foods so remove the choice aspect, that should definitely help!
ONE big thing lettuce has no nutritional value. Stick to dark leafier greens and a range of colourful veggies.
If he's out of the cage so much why not try implementing meal times... a routine is very important for the bird.
Also please get him checked by an AVIAN vet, its important just in case and it helps makes sure there's nothing wrong to start with. From the pictures he looks healthy enough but its very important that you do it as much as you'd take a newborn baby for its first check up
