Sounds to me like a dust cough.  The barn has probably been more closed up for the winter, and your horse probably hasn't been as active.  I agree with the wetting the hay, it can help.  I would also try to allow plenty of turn-out time so he can clear his lungs.  If it doesn't get better as the weather warms up or if it gets any worse, I would check with the vet again.

Ditto ColtysHeart.

One of my horses developed a dry cough after we moved to Middle Tennessee.  I started wetting his hay down when he came in at night and it was 95% efficient.

When my 20 yo TWH was diagnosed as being EMS, I had to start locking him up at night, instead of letting him have the run-in stall attached to the 100' X 24' paddock.

I switched him with the dry cough horse.  That was one year ago.  The dry cough horse (and the others) still get all their hay wet down, but this horse has not coughed once since I got him out the stall.

The EMS horse that is in that stall has never coughed, so evidently Mr. Dry Cougher's lungs were just sensitive enough that being in that stall all night affected his lungs.

My stalls are pipe panels, there is way more than ample ventilation and I clean stalls every day, so it was just something about not being able to move around in the fresh air that bothered him.

So again, I totally agree with the hay wetting and as much turn-out time as he can have

Another last minute thought:  There might be something in the hay that he is sensitive to.  In general, the 2007 hay certainly wasn't a bumper crop and hasn't been anything to brag about.

You will likely here a million horror stories about COPD etc but to be honest if this was my horse I wouldn't give it a second thought. It is probably a mild allergy to something in the air, the bedding or his hay.

You could soak his hay if you are particually worried, or even try him on haylage but we have a chestnut gelding at my yard who, in winter, ALWAYS coughs about 8 times during the first 10mins of being ridden. After this he is 100% fine. We've always ignored it as there was no discharge and he didn't seem to be in any discomfort and the cough went away. However his potential new owner was worried and called the vet to check him over.

Vet could find nothing wrong and basically told her not to worry, just to keep a general eye on it incase it worsened and that as long as he was happy in himself it was probably just an allergy...

that is my experience, I always believe that if there is no serious discharge, horse has normal temp and is behaving in his normal way, there isn't too much to worry about! JMHO

My arab seems to develop a cough each spring when she is inside due to wet and muddiness outside.  I think it is the dustiness of the hay and the fact that she is inside with little exercise.  It usually clears up once she is back outside and can start on grass.  The only problem I have with this is that I can't really ride her much.  If he is only coughing twice per 2 hours or so I wouldn't worry too much.  But soaking the hay and more daily turnout would probably help.

Thank you to everyone who replied to my coughing Cruizer.  By the time I got done reading all your replies, the cough was gone! Turns out it was just a cold.  Still learning,,,I will figure it out!,,,thanks again!

Having been through this twice, I can safely say that it's not a bad idea to have the vet check it out.  Our gelding simply coughed for no important reason other than to clear his passages prior to work, so we didn't need to treat him and he lived to a ripe old age.  My mare, however, started with a mild case of rhino and developed a cough.  The cold wasn't a big issue, but the cough persisted for a while so I had her allergy tested.  She turned up positive to 22 plants in her environment--classic pasture heaves--due most likely to the irritation from the cold.  She's been on Tri-Hist during allergy season ever since, and she's doing fine.  But if I'd left her untreated, she might well have developed permanent damage. 

I figure it's worth the vet call and the blood work to avoid long-term care issues.