i know what you mean about seeing him fall over, you want to help him so you can try these 3 things-
1. you said the 5 year old rooster came to you recently, so ask the previous owner if he was vaccinated for mareks. this may give you some level of comfort? although they say a small percentage of vaccinated birds also get it.
2. also ask the previous owner if the bird has stumbled all his life, or if he noticed the bird stumble ever...
3. seek an appointment with an excellent poultry vet in your area.
just remembered another very remote and maybe (non poultry) medical explanation for what you describe
***there is some human condition (not stroke) that occurs when people stand up quickly and i dont know if it is a heart issue or artery issue or what but for just a moment there is not enough blood to the brain and it causes you to lose your balance, fall over/faint just momentarily. syncopy? yes thats it-
http://www.heartcenteronline.com/myheartdr/common/articles.cfm?ARTID=363 i can see it now, your vet putting the rooster on a mini tread mill with ekg wires hanging off him and a pretty hen just in front of him to make him run...then off to the tilt table, the blood tests...