I am very sorry to convey that there is no cure for a choroidal coloboma available yet. You can get the best possible vision from that eye, by getting a good refraction done by a qualified Ophthalmologist. If your vision drops further, it may be due to cataractous changes that occur earlier in a colobomatous eye. In that case, a cataract surgery may restore your vision to the precataract level.
Next Steps
Consult your Ophthalmologist regularly, and immediately if you notice a sudden change in vision.