My first thought was to see if I could locate any photos of members of the 14th. Searching the index of Military Images Magazine, I came across only one published image. It was in a special Illinois issue, November-December 1990, (volume XII. no. 3, page 13). While the pic is of a private, and he is not showing headwear, the caption might be of interest to you, as it might give an idea what your ancestor might have experienced.
"Private Newton Smith, 14th Illinois Cavalry, rode with General George Stoneman's raid to free Andersonville prisoners in July of 1864. Soon surrounded outside Atlanta, Stoneman surrendered. Smith's brigade, however, chose to cut its' way out. Smith didn't make it. He spent the rest of the war with the prisoners he set out to free". (( Newton was in Company A, and was promoted to corporal, but that date is unknown)).
As to Yankee headgear, most group photos show privates and non-comms wearing the same style. Officers with a commission often, but not always, wore a different style. Many western units were equipped with a slouch, but I can't say of the IL 14 Cav was one of them. I'll do some more looking.