Tonics etc. sold over the counter are not usually that great. They are not designed for chams, the ingredients may be who knows how old, and there may be forms of vitamins you don't want such as preformed vit. A which chams are quite sensitive to.
Give us details about your setup first. Chams, like most basically wild creatures will hide signs of problems so as not to appear easy prey. Your cham may well chase females etc. up to the point he's really too sick to care. Review the basics, because if there are other husbandry issues no tonic will solve the problem. It will just keep recurring. A better gutload will do much more good than a tonic over the long term. You say he looks dehydrated? What is your watering/misting routine? What's the cage humidity level? A dehydrated cham may be even more sensitive to vitamin overdoses and damage to kidneys.
So, let's see if we can figure out what the problem may be first.