You could certainly try the 'soy meat' on him. It'll add some good quality protein to his diet if he eats it.

"I wish I could buy small portions of meat or chicken just for him, but I can't"

You could look in the baby food aisle in the market.

Another option would be to buy the smallest portion of chicken/meat that you can, cook it, chop it up, and freeze it so that it's not stuck together in a big solid block. You could freeze it in a thin layer on a cookie sheet, or in an ice cube tray to make small portions. Then keep the loose pieces in a freezer bag and take out the amount you want when it's serving time.

Baby food is a good idea. I will have a look.

Yes, I know freezing would be a good idea too (I freeze a lot of stuff), but for a vegetarian having dead animals in the freezer is not very appealing... It's a psychological rather than practical question for me.

I can really understand the dead animal in the freezer thing. I'm not a complete vegetarian, although moving more and more in that direction. I do not cook any meat at home. I do have it at restaurants or at someone else's house.

I'm a vegan, but I always say I'm an unfaithful vegan. I'll still eat meat if someone makes it for me, but if I'm on my own, I cook vegan... frankly, I don't know what to classify myself as, but I get it.

Beans are a great source of protein and are animal free.

Parrots don't require animal protein at all so there's no need to provide it. But if you want him to have a little now and then as a treat you can scramble an egg, chop it up, and keep it in the freezer just like the suggestion for the meat. Grocery store eggs aren't even fertile so there are no dead animals whatsoever involved in it lol. My birds get a small amount of egg twice a week and they love it. If you don't buy eggs for yourself, maybe you could buy or "borrow" a single egg from someone you know.

Ah, for some reason, I thought this entire thing was regarding protein. Don't know where that came from.

I am vegetarian, not a vegan, although I am trying to become a vegan, so I do eat eggs myself and he usually gets some. I also give him egg-and-biscuit formula.
For vitamins by the way the vet once told me to plant some seeds, like wheatgrass, and it seems to work better than veggies because he likes to play with the leaves and the stem before eating them...

Cockatiels mostly eat grass seed in the wild, and it's reported that they chew on the grass stems too. So wheat grass (and lawn grass too for that matter) appeal to his natural instincts. A lot of cockatiels will respond better to long thin vegetables that look sort of like a stem than to cut-up vegetables in a bowl. Things like carrot tops, cilantro, and asparagus. Leafy greens that have been clipped to the cage to look sort of like a plant is growing there also tend to be appealing, and veggies that look sort of like a seed or seed head, like broccoli florets. My guys like to eat their veggies foraging style.