Does anyone know what college he played basketball at?

On the Criterion Laserdisc "The Addams Family", they have lots of frames with photos and text. This includes biographies of the cast. Regarding Mr. Cassidy they say:
...grew up in Philippi, West Virginia. Although his prowess in high school basketball and football might have suggested otherwise, he preferred acting to athetics, and his participation in high school plays planted a seed that would take root years later. He earned a degree in speech and drama from Stetson University in De Land, Florida, having previously attended Wesleyan College in West Virginia.

So I am going to go out on a limb and say the answer to your question in probably nowhere. If he had played basketball in college I think the careful folks at Critereon would have mentioned it in their bio of him.

But they are only human, and if they missed it, he must have played at one of the two colleges he attended, with Stetson being the more likely choice.

Ok I can answer this, I read an article in the Atlanta Journal-Constitution at the start of the NCAA basketball tournament and it says he played at Georgia State University.