Actually he is a betta imbellis, very similar looking to the plakat but not as big of a fighter. Plus the fins don't come to as much of a point as plakats. I have actually heard of two males being kept together in a ten gallon.
You should feel very lucky! Imbellis are rare and white ones are even rarer still. Try to find a female and you could make a lot of money with a pure strain of imbellis!!!!!!!! Breeding him with a splendens wouldn't make that good of a line imo... save him for a special female.