Hi there, do what you can to limit light as per everyone's suggestions. Temperature is a bit trickier though. If the sore on his back is fungal or bacterial, which it does appear to be, increasing the temperature will make it grow faster.
You should be able to get rid of the velvet with daily 100% water changes and 3tsp/gal aquarium salt (with a gradual increase in the concentration as you have been doing) without increasing the temperature. The aquarium salt should treat the sore as well if we're lucky.