He could have a respiratory disease, either viral, bacterial, or fungal. How old is he? Did he have symptoms of coccidiosis a few weeks ago when you noticed his diarrhea? Symptoms are lethargy, weakness, standing puffed up, diarrhea (with or without blood,) and poor appetite. Has it been wet and rainy where you live, where mold and aspergillosis may be a problem? His symptoms sound similar to aspergillosis or other respiratory diseases such as infectious bronchitis, MG, coryza, and ILT. Here is a good link to read about these diseases:
http://edis.ifas.ufl.edu/ps044
http://www.thepoultrysite.com/diseaseinfo/7/aspergillosis
http://www.aspergillus.org.uk/indexhome.htm?secure/veterinary/Poultdis1.html~main
Aspergillosis/ brooder pneumonia/ mycotic
pneumonia: When the source of the disease is the
hatchery, the disease is called brooder pneumonia.
In older birds, the disease is called aspergillosis. All
birds (domestic poultry, pigeons, canary and zoo bird
species), animals, humans, and plants are
susceptible.
Aspergillosis occurs as an acute disease of
young birds and a chronic disease in mature birds.
Young birds have trouble breathing and gasp for air.
Characteristically, there are no rales or respiratory
sounds associated with aspergillosis. Feed
consumption decreases. Occasionally there is
paralysis or convulsions caused by the fungal toxin.
Mortality in young birds averages 5-20 percent, but
may be as high as 50 percent. Mature birds also
have respiratory distress, reduced feed consumption,
and may have a bluish and dark color of the skin
(cyanosis). Nervous disorders, such as twisted
necks, may occur in a few birds. Mortality in mature
birds is usually less than 5 percent. Aspergillosis is
caused by a fungus. The fungus grows well at room
temperature and higher. All litter and nest materials
(peat moss, peanut hulls, sawdust, peat, bark, straw)
have been known to have been contaminated with
aspergillus. Feed and water should be suspect when
attempting to identify the source of contamination.