Ok for us to help you were gunna need you to answer a few questions.
What size tank have you got?
Have you cycled your tank?
Is the tank heated/filtered?
What are you ammonia, nitrIte and nitrAte readings?
Do you de-chloronate your water before putting it in the tank?
Is there any discolouration to the fishes scales/fins?

Is the tank heated? And water perameters would be very helpfull. Scales can go paler due to stress. How big is your tank and what ilter are you using? I'm sure wel find out whats wrong with the little guy.![]()
Have you any idea how old the little guy is and has he been pooping at all? This may sound odd but it could be contipation. And try to get a water test kit and check your perameters, API do a master kit that includes everything you need and most people on here swear by them.when he darts around is he glancing off objects in the tank? scratching on things?
dullness can be casued by over production of their slime coat due to some sort of parasite. if you see no spots (like grains of sand or salt) than it is not ich. shine a flashlight on him when the lights go out. do you see any gold dusting?
is he bloated? is he eating still?
when in doubt do a water change. see if that helps him out a bit.
is he floating as tho he cant go lower in the tank? or just hanging out at the top? if he's having trouble maintaining balance or finding it hard to swim down then it is a swimbladder issue. try feeding a cooked, deshelled, cut up pea and see if that helps.
good luck.

defo give him a pea (they love them)...test the water...regardless of stats do a waterchange. loads of times waterchanges will make them feel better just by doing one. try to do one every day for the next few days and see if that helps things.
if he was scratching you'd notice. it is a very fast "flick" agains objects or the glass in the tank. its pretty obvious. rsting, even for long periods, is quite normal for many bettas. have you recently put anything in the tank? plants or decorations? other fish or inverts? he might just be moping a bit until he gets used to his new surroundings.
good luck.
sounds like Ich. do a waterchange and raise the temp in the tank if you are able to 86 degrees F(some heaters are not adjustable)...add 1 tsp/5 gal of aquarium salt and get an ich remedy. this parasite is often very easily treated.
good luck,
