Thank you for taking this little guy in! Sounds like it`s goind to take a lot of work to get him back to a `normal` happy, cockatiel frame of mind.
Have you taken this bird in to see an avian vet yet? Having had only one perch for 10 years, it`s very likely that this bird has arthritis in his feet. With some pain management you may find him moving around the cage more in next to no time.
Hopefully you have a variety of perches in his new cage? Having only dowel perches is one of the main causes of arthritis in birds. They need perches with a variety of textures and widths, and uniform size of a dowel prevents them from exercising their feet and keeping them healthy. Natural wood perches are great, and I bet he`d find a rope perch very comfortable. He`d probably also highly appreciate some sort of ledge or platform to relax on. You can get a wire platform and wrap it in a substance called `vet wrap,` which would make it more comfortable for him, or you can check the rodent section of the pet store. They sometimes carry wooden platforms for chinchillas or animals like that to use.
In fact, this cage sounds big enough to accomadate a `boing` (a spiral-shaped rope perch that hangs from the top of the cage). The swinging and boucing of a boing is a great way to get him moving and exercising those muscles again. Remember, this bird is disasterously out of shape!
Its also important to slowly introduce a wider variety of toys. A good starter toy is called `Shredders.` They`re woven palm leaves, and birds just love chewing on them. You can weave them through cage bars, wrap them around perches, or wrap it around a piece of millet spray. (Speaking of millet spray, that`s a great way to get the bird to explore new parts of the cage. I`ve never seen a cockatiel that could resist the temptation of a millet spray!)
Since he hasn`t had much exposure to toys, add them one by one over time to limit the stress. You`ll need: chewing toys, preening toys, noise making toys, knots to untie... Most cockatiels enjoy chewing on popscicle sticks and wooden icecream spoons. A toy with a bit of frayed rope at the end can encourage preening. A toy with a bell on the bottom incorporates the noise. A toy made with lanyard string, strung with colorful beads with a knot on the end is also a winner with some birds. Untying that knot on the end makes all the beads crash to the floor. Always fun!
Since this bird has had little exposure to toys, play with them in front of the bird and pretend to have a wonderful time. Then perhaps hid little bits of millet in some of the knooks and cranies of the toy, or tie it to a piece of millet. That`ll encourage the bird to start exploring the toy.
It`s gonna be a long and slow rehabilitation. But start with a visit to an AVIAN vet. Goodluck! And please come back here with all of your questions.