So far you've done all the right things in making him more comfortable and less likely to hurt himself with a fall.
17 years is into the 'golden years', a geriatric bird.
As unusual as it may sound, when a bird loses its balance, it is usually the result of an internal problem.
Let’s go over a few of the more common:
Sometimes a tumor on the kidney will not appear on the outside of the body, but other symptoms such as limping, the loss of use of a leg (or both) and/or imbalance might occur. This happens when the tumor presses on certain nerves.
Tumors can also be in a male’s testes or female’s ovaries and there are not always obvious changes until later on when the growth is more dominant inside.
Other indications that there may be tumor activity would be a change in cere color, weight loss, changes in droppings (often becoming pasty, soiling around the vent) and just subtle, overall changes that owners may sense more than actually see or be able to describe.
Fatty liver disease is something that is often seen in a bird on a seed only or predominantly seed diet. No matter how much the manufacturer insists they are fortified and healthy, they are misleading all of us.
and though a cockatiel site, this will apply to all psittacines (parrots)
Skeletal problems, deficiencies and even toxicities can cause a loss of balance and restlessness in some birds, as well as the more common symptoms such as breathing difficulties, open mouthed breathing and so on.
Zinc and other toxic metals or substances can be ingested slowly over time when toys, clasps, chains, links or even cages are chewed on or played with. Other poisonings occur when the bird actually swallows a toy, link or piece of one. Watch out for bell clappers for instance.
Take a look here under critical conditions to reassure yourself that urgent veterinary intervention may be necessary (these symptoms apply to all birds, not just ‘tiels)
Vitamin A/Beta Carotene may help produce some improvement if this is related to a deficiency. It's certainly not going to hurt to try.
However, it must be administered with natural food sources if done at home.
Vitamin A is most ideally received from natural foods like sweet potatoes, yams, carrots, squash and other dark colored vegetables. If your bird doesn’t care for fresh vegetables, a ½ teaspoon of natural baby food (human baby food) of any of these vegetables. Again, it must be all natural and nothing but the vegetable with water sufficient for processing.
If this were my bird, yes, I'd see a vet about this. If for no other reason than to know I did everything I could - and in a best case scenario, this might be something treatable and able to overcome.
You've done an excellent job in getting him to this age. I wish you only the best