At eight weeks old it is too young to be eating much apart from small live foods ie brine shrimp, live daphnia. Personally I would never have dreamed at selling a fighter at that age. Where did you get it from? What were they feeding it?
You could try chopping up a single defrosted bloodworm and feed it a couple of 1-2 mm pieces. Apart from that see if you can get hold of some decapsulated brine shrimp. If you have to go dry foods try the micro pellets. Leave the fighter in the container, it will get used to it - it will be totally freaked out being so young and placed into a tank with other species of fish.
As a side note eight weeks you are unlikely to be able to sex it properly, it may end up being female