I have a user of Windchill who needs to use assistive technology as he can't use a mouse. I was hoping that there is a way that he could navigate Windchill either using voice activation software, or use the keyboard to navigate Windchill.

Well my thought process here was to get a touch pad tablet device and connect it to your network, as iPad and similar devices would still be able to access Windchill though the browser native to the tablet.  While some functionalities may have issues, the user would still be able to do any activities housed within the web browser itself (workflow task activities, administrative tasks like context team setup depending on the user's ACLs, check out/check in though modification of the object content depends on the tablet's abilities too, like a MS tablet modifying MSWord docs, etc).

Creo or other CAD modification likely is out due to software compatibility.

Maybe not the best solution you'll get on this thread, but something to think about.