It's very necessary, they ar.en't hardy at all to be honest. Very sensitive to water parameters because of their delicate fins. I'd get test kit ASAP but in the meantime do daily water changes to stop his fins from deteriorating any more. As for medicating him, Aquarium Salt, Stress Coat, Indian Almond Leaves and a close eye on water parameters should do the trick. But main priority is daily water changes and test kit. I can see the picture and usually fin rot is categorised by blackening around the edges of fins, so hopefully it doesn't look like this. Best thing you can do is research, research and do more research on how to take care of him PROPERLY and have the best environment for him. The guy at the pet shop horribly misinformed you as he is probably also misinformed on the care that needs to go into Bettas. But yes, research.