it doesent matter how old he is ,he is going to get huge no matter what.it is best you return him.i'll ask how old the shark is. there were like 6 in a 20 gallon tank, maybe they are smaller ones.
also are there any other symptoms your betta has? (discoloration,bumps,etc)
because i think i can see white spots on his pectorals or it may just be the pic.
thanx i now see how you have the bowl rigged.he is definitely clamping his fins