If you Google "equine slobbers", you will get a ton of hits on the subject but, this is a pretty good article. Your Horse’s Strange Slobber
Where it says in part:
--in the spring and summer, clover contaminated with Rhizoctonia leguminicola fungus is the usual trigger of what is commonly called "slobbers."
Affected clover is easy to spot: The leaves change from vibrant green to a grayish tinge and eventually turn black. When consumed in moderation, forage infected by this common fungus is harmless, but its bitter taste irritates horses' salivary glands. If you find clover-induced slobbers distasteful, your only recourse is to take your horse off clover-containing pasture until the fall.--
If you're still not sure, or the slobbers persist longer than a week or so, call the vet, just to be on the safe side, since there is always the slim chance something else is causing the horse to drool.
However, as long as the horse does not seem depressed, is eating, drinking and recycling all that in a normal fashion, he's most likely dealing with disgusting cover slobbers.
We have been lucky, so far, this year. We've had so much rain and have had to bushhog so much, the clover that is in the pasture gets cut frequently, so I haven't dealt with slobbers------------------yet
Also, some horses are more affected than others and will drool by the buckets, while some barely drool