<p>First thought is upper respiratory tract infection (URTI). This is a very informative site I found for you....rather than typing it all out... <a href="http://www.cathealth.com/UResp.htm" target="_blank">http://www.cathealth.com/UResp.htm</a><span style="display:none;"> </span> As you have a multicat household I would definitely bring him to the vet. Could be a virus (simple or much more serious) A congested cat cannot smell very well and cats that don't smell well will often go off their food....this, of course, can lead to many more problems. Unlike most dogs cats going off their food can lead to trouble quick. </p>
<p>The vet will do a full physical and <em>may</em> run blood for Feleuk/FIV and a general blood profile/CBC to rule out any underlying causes. Antibiotics/eyedrops may be prescribed to help. </p>