Only certain counties have a 10rd mag cap like Cook. Its a problem with many online sellers that think the entire state is banned from having anything over 10rds yet its only a very small amount of counties and not even all the towns in the counties are restricted because of home rule. Midwest Sporting Goods is in Cook yet because they are in Lyons with a home rule they have no mag capacity restrictions in that town.
Where in Illinois are you OP? Best bet would be to take your dad to a place that rents guns like Midwest Sporting Goods, Megasports or GAT. Suggestings for a bigger barreled 9mm would be Glock 17, XD Tactical, Beretta 92FS, Browning High Power, CZ75 and many more. If you want what fits him best then renting is the next step. I know Midwest for sure lets you rent a gun, shoot it and then pick out another gun to try out over and over no matter how many guns you want to try out at no extra cost.