One cheek is drooping down, will Botox help?

Botox can work on lifting the brows by injecting the muscles that pull them down. In the corners of the mouth, there is a muscle that pulls them down (DAO) and Botox can be injected there to avoid the excessive action of this muscle on that area. Another muscle that may pull down the area along the jawline is the Platysma muscle, so Botox can be injected in that muscle to avoid too much pulling along the jawline. Other thing that can be use to improve the contour of the lower half of the face is Botox injected in the masseter muscle when this is too big. Some Botox will decrease the thickness of the muscle on that area, making the lower face look more slim. Fillers are also helpful to improve the contour of the cheek area, by filling some specific areas, you can get a little more cheek projection that can work as a lift. Please consult with an expert in this particular field, get a consultation in person to get an evaluation and the options for your case.

No-botox won't help. See a very experienced surgeon who can discuss options since you do NOT WANT IT TO BE WORSE!

I do think masseter botox would help your facial contour.  Generally the dose in in the 12 to 25 unit range.  Treatment seems to last about 6 to 12 months.  I would not recommend adding volume to the lower cheek especially with fillers that can't be erased.