And when you do water changes NEVER change more than 50% of the water at a time. It is best to do freaquent SMALL water changes no more than 25%. This way you lower the risk of killing your fish from a large pH change or a mismatched temperature change. Let the new water stand for a day so it becomes the same temperature as the water your fish is currently in and treat it to remove the Chlorine.
TOTAL AND UTTER RUBBISH THIS PARAGRAPH - IGNORE IT
I cannot express how important this is. It is the main reason why new fish owners kill their fish unintentionally. Do small water changes and it's a good idea to get a pH measuring kit so you know what the pH is and if it is dropping too fast.
If the pH drops within a few days, it means you need to do more freaquent water changes.
PH is the least of his worries!! It is unlikely to drop anyway!
The advice you were given above about putting him in a tank of 3 to 5 gallons minimum is excellent. And in that small of a tank, I would not look to add any other fish or you'll need to do daily 25% water changes.
Take our advice and your Betta will love you and will even interact with you if you spend some quality time with him every day. A betta can bond with a human much like a dog does. Treat him like royalty because..... HE IS!