Help new oscar sulking,not eaten in 3 days!

help, my new red tiger oscar is just sitting on the bottom of the tank,sulking.
he hasnt eaten in 3 days...the first day i got him he ate a pellet(simple protien pellet)but since then he hasnt touched any of his food, he just sits there at the bottom of the tank he seems to be breathing lightly and moves around the bit of wood he has to hide behind, but spends alot of time just sitting at almost a 45degree angle. his belly is starting to look a bit empty
what can i do?

well...i have no ammonia/nitrite/nitrate tester...as bad as that is...i have 8 tanks and ive never really needed one cos ive always established the tanks then slowly added fish and ive never had a problem...no excuse for not having a test kit still i know .the ph is just to the acid side of neutral and he is in the oldest set up tank i have (my fist one) so it is well established...
he is in a tank with a divider in it,on the other side is australian rainbows he semed to enjoy looking at the rainbows but he kinda frieked them out..the rainbows are still happy, just not as happy as usual. the oscar is approx 2 inches long

yeah all my lfs stock ph kits but they never seem to have amonia/nitrite kits. i tried frozen food but he still seems to sulky...then i tried a small mealworm, and he looked at it cos it wiggled but seemed determined to sit in his spot.
went back to the lfs i got him from, asked what they fed him....GOLDFISH FLAKE >< ! still...he is sulking and would not accept that either, the one thing he ate was 2 small protein pellets, but hasnt seemed interested in them since

also.. could trying to give him many different foods(to see what he will eat) just be confusing him?...i have tried protein pellets,shrimp pellets, cichlid crumble (the neolamprologus brichardi loves this),cichlid dinner(frozen concoction),meal worms and tropical flake *phew* should i try to mix it all together?,like blend it and add gellotine?or should i just stick with the one (prolly the protein pellet cos he ate that once at least )then try and vary his diet cos im not sure if the protein pellets will be good for him in the long term especially if he has been fed goldfish flake cos he might now be viatamin/mineral defficient..and the protein pellets are the cheepest food i got which leads me to believe its made of low quality ingredients (i had it only to feed my apple snails)