Hi, welcome to the board.
Could you answer the questions located here:
http://www.redearslider.com/forum/viewtopic.php?t=2142
That will help us to better assess the situation. Also, photos never hurt!
Are you sure he is a "he"? Does he have lovely long claws? If not, if he might be a she, she might be gravid with eggs or one may have exploded, which would require immediate vet care. If he is a she, she might need to lay eggs. Has your turtle been digging with his hind legs lately?
Back on topic, has he been lethargic in the water or on his basking area? Has he been basking well lately? Are there any mucus bubbles coming from his nose or open mouth breathing? If you have seen open mouth breathing, he needs to be taken to a vet immediately as he likely has a respiratory infection, which is only treatable by the vet.
Please give us more info so we can help better!