I think my goldfish might be almost dying.? :'(?

well, he's at the bottom + very still and breathing. his top fin is down + he looks sooooooo unhappy. it seems wierd, because i can see him now + he is swimmin around a bit when im not there. but then when i come he is all still + quiet. he has NEVER been like this before. we have had him for about 4 years now + got him from my old bestfriend who is now in london. he used to have another fish with him but then that one died of age + he hasnt been acting the same since he/she died. (about 2 years ago). but he has never been like this.