Alright lets go over the basics...
How long have you had him? Because if you just recently got him, his colour could changes as he becomes more comfortable in his home. A happy healthy betta will be deep rich colours.
How often do water changes? Because for a tank that size, water changes should be done at least every 2-3 days.
Have you tested the water parameters of the water? Such as ammonia, nitrate, nitrite. You can take a sample of your water to a Petsmart and they can usually test it for free.
Finally, does he display any odd behavior? Like listlessness, heavy breathing, darting around, clamped fins, constantly hiding, etc. Those are usually signs of a sick betta.
Also... I would strongly NOT recommend a heater for that size of tank, it can EASILY overheat it and kill your fish if it's not calibrated properly. 80 degrees F or 25 degrees C is the BEST temperature you should keep a betta at. A little over or a little under is okay, but usually keep around that.
I'd also recommend you getting a bigger tank if possible. 2.5 gallons minimum. It's safer to put a heater in a tank that size than it is to put in a small one.
In any case, if you're still concerned, I'd recommend adding aquarium salt to his tank. About 1 tsp per gallon is good. Dissolve it first in treated water before SLOWLY adding it to his tank or else the salt crystals/the sudden change in the water could hurt him.
:3 Aquarium salt is a natural good ol' remedy that helps promote better health and offers a little defense against illnesses, simulates the natural production of a fish's slime coat, and will promote better gill function (which is good in the case he might have something like ammonia poisoning).