It is quite possible he can smell a bitch in season. My dog always goes off his food and I can take him for a 3 hour walk, gets back and is screaming to go out again 5 mins later!!! This is ONLY when he can smell a bitch in season. Another good sign is that when I take him out he charges round the walk, nose glued to the ground.
His eyes become kind of glazed, I can always tell when the bitch is getting near the end of the 'season' as he then starts to try and mount me!!! My dog is the size of a Great Dane!!! I have been known to tie him to a fence and phone my partner to come and get us as there is no way I am attempting to walk this dog through streets home!!
Don't, whatever you do let him off the lead while he is like this - you will never get him back!!
It only lasts about three weeks, then you will have your normal, loveable dog back!