My fish is on its side at bottom of tank not eating! help!?

He is in a decent sized tank and is a comet goldie. He's about 3yrs old and a lone fish. He's been ill since Thursday. I've only just got round to a water change with the de chlorifier stuff in and have tried to feed him a thawed out pea chopped up but he just spat it out. He's on his side now and nt swimming at all :-( is there ne such thing as a miracle cure!? Help please what else can I try? Seems to me he's a gonner ne way so I may as well try. Please ne one?