Thanks for the extra info. Obviously I can't tell you 'for sure' what's wrong with your little guy via the net, but I can point out some possibiities and encourage you to pursue them based on what you're seeing (and your own instincts).
As unusual as it may sound, when a bird loses control of their feet, grasping ability or balance, it is very likely the result of an internal problem.
Many owners will be convinced their bird has somehow broken their foot or leg when they see these symptoms of inability to stand (or difficulty), perching problems, loss of balance or holding their leg/foot clenched. Sounds exactly like what you're seeing doesn't it?
Let's go over a few of the more common causations:
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.
Renal Adenocarcinoma may invade the ischiatic nerves and constrict them, causing (disuse) atrophy of one or both legs.
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. If you are not seeing other symptoms at this time, you have probably caught this early enough to save your bird's life.
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.
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.
A surprise to many owners is that a crop problem can be behind the symptoms too. Anything that contributes to an electrolyte imbalance/nutritional
As with all things that might go wrong with our feathered friends, early intervention gives us a better chance at keeping them around a bit longer.
Blood chemistries and X-rays should be expected (and encouraged).
I know how you feel about the trauma of a vet visit, believe me, I completely understand that. Some of my rescues think a hands on exam is going to kill them! The thing is, without a hands on exam... it just might .
Take a look here for more about fatty liver disease and many of the symptoms to watch out for:
It's generally recommended that most of today's companion birds have a predominantly pelleted diet. Pellets have been continually updated since being introduced to the market years ago and today's formulas are better than ever.
Supplementing this diet with fresh foods every day is ideal and many owners find they can re-introduce seeds - in limited amounts (perhaps once or twice a week) without the bird refusing the pellets overall.
http://www.cockatiels.org/articles/nutrition/diet.html cites feeding both seed and pellets, but only after weaning the ‘tiel from a mostly seed diet.
By making these changes now, once your little one is diagnosed and treated for his current condition - you can probably avoid any future problems like this and keep them around for another decade or more. Let's hope, right?