Trying to get better armor for my follower, but he doesn't take it off no matter what and I don't really understand leather armor enough to know what's better, so... please help

see the stats? o_O

Whatever armor you want them to wear, power it up through smithing so that it has a higher defense rating than whatever the base defense of the follower's default armor is. Followers will only equip armor that has a better defense rating than their default.