Betta's require very simple things, a tank, a heater, conditioned water, food and decorations.
Some may view a 2.5g as small, if you believe he is happy in that sized tank then providing you keep up with water changes it will suffice. Without a doubt however, Betta's require a stable and warm environment. This environment is best replicated by use of a heater.
You could get an adjustable heater or a betta heater. Preferably an adjustable heater as it maintains a set temperature and will turn itself off once that temperature is reach, and turn itself back on when it drops below. The issue you may find is that the smallest 25w adjustable heater may not fit your 2.5g.
If this is the case your other option is to purchase a betta heater. They are usually anything from 5w-10w submersible heat pads. The downside to these are they cannot be adjusted and they do not switch themselves off. Generally they are capable of heating the tank a few degrees above room temperature.
If your location however remains humid and warm all year round, meaning the tank temperature is within the acceptable betta range. Then you won't require a heater - but outside of Thailand/Malaysia and neighboring Asian countries - this is most likely not going to be the case.