homeopathic remedies work on the basis of like cures like. So, if you have a fever you take something that gives you a fever and you're cured! (AKA the laws of similar)
It's a bit more than that. It's based on the idea that if you take something your body reacts to it to try to create homeostasis so that when you reduce the amount you have the opposite reaction. For example, if you took a laxative every day and then you reduce the amount you take drastically, you would become constipated, because your body treats taking the laxative as your "normal" state. The same kind of thing happens when people take nose drops for a long time.
There are some flaws in extending this to the theory around homeopathy but the above has some sense to it.
If he just has a cold, giving liquids (I assume the medicine is liquid) and some attention is probably as good a cure as any. If the medicine has a nice smell, it probably helps, too. Feeling good emotionally can help boost your immune system which can help you get better physically,
My mother used to have a saying - if you take medicine, a cold lasts seven days. If you don't take medicine, it lasts a week.
ETA: About the fever thing, that's kind of what DH says about drinking hot tea in hot weather. You drink hot tea, you sweat, you cool off. It's all about working with the body's natural reactions.