By the sound of it he is getting swim bladder disorder, this is when bacteria invade the swim bladder the fish uses to remain bouyant and makes it difficult to swim.
The first treatment for SBD is to use Epsom salts (1 teaspoon per 2 gallons, upping the dose to 1 per gallon for medicinal purposes) which acts as a natural laxative and organ cleanser and should hopefully purge any guests in his GIT that are breeding the bacteria. Usually they betta will pass the blockage and the swim bladder will go back to normal.
If he does not appear to get better with this safer method or even begins to float more it is time to move on to antibiotics to kill the bacteria directly. I prefer tetracycline myself but the US has oher types like kanaplex available.