Sounds like the beginnings of fin rot. I couldn't really see anything in the pics. If fin rot is caught early enough, which I think it has, just be sure to keep his water extra clean and also pick out uneaten food. Try to get some blackwater extract, which can be found at PetCo (the nearest to me keeps it in stock), and add some extra to his water (the water should be REALLY brown). This method has worked for me in the past. Also, since this guy is a CT (Crown tail), his water should be kept clean to ensure his fins stay healthy. CTs, HMs (half moons), SD (super deltas), and other long, big finned bettas need a little more care than PKs (plakats).