What is your temperature? It should be in the 86F range. This temperature will have a poor effect on your neons, as IME and in my reading I have found them not to well at all in temps above 80F. If your temperature isn't around 86F, then you should bump it up... then you will probably see an increase in appetite.
Sublime is right, discus should never be kept alone, because they are a shoaling specie. Juveniles (under 4.5'') especially do poor alone. 3 individuals is the minimum, 6+ is highly prefered, but you can't do that in your 40g. I personally would pick either the current stocking, or the discus because you do have quite a few conflicting temperature requirements.
Next, I highly recommend going into the article section and looking at the "Beginners Discus Article" as it will help you immensly and get you started. You may also want to skip over to DAAH (
http://discusasahobby.com) which is a forum dedicated to discus.
IMO, your tank is too small for any other fish besides a shoal of 3 discus... you may be able to have a small shoal of 3 bronze/sterbai cories and 3 otos but that's about it. If you wanted to drop some money and buy a bonded pair (they don't have to be a mating pair), then you could have just the pair (your current discus would have to go though), then you would have space for a shoal of tetras (glo-lights are great with discus, BTW : ) ) like cardinals, pencil fish, or any other heat tolerant shoaling fish.
HTH