First, move him to a hospital tank and bring the temperature in the hospital tank down to 68 degrees F (don't lower it more than 4 degrees in 24 hours). Then, answer these questions to give us an idea of what to suggest next.
Can you see any bubbles on your seahorse (especially on its tail)?
Does the pouch look bloated?
What is he eating? And how much is he eating?
What are the parameters in the aquarium (temp, pH, ammonia, nitrite, nitrate)?
Any recent changes in the aquarium?
What kind of setup is he in (tank size, flow, etc.)?
Who are his tankmates?
Where did you get him, how long have you had him, and is he captive bred or wild caught?
The head down, tail up, sounds like some form of gas bubble disease, but then he'd be floating, not laying on the bottom. The white pooh suggests he is either not eating or has parasites. More info will help us let you know what to do next. You also may be able to find help at
www.seahorse.org, they have a seahorse disease pictures forum, and also a seahorse diseases and treatment forum, that may help you out.