The spring grass is really starting to come through here now- that sounds like the most likely culprit!
I'm a huge fan of probiotics for horses tummies- gives them a bit of extra help. I use Dengie XP as and when but 'human' probiotic yogurts and drinks etc do just as well apparently.
Cant for the life of me see how anything living in a human probiotic could make it to the intestines without being destroyed by the acid on the way down....Id stick with one made especially for horses. Look for MOS/FOS on the label.
Put a good dollop of natural yogurt in his tea! Most likely cause is spring grass (ours are the same - they've been on the same (large) paddock all winter but the grass has really sproated the last few weeks and their poos are somewhat undesirable!)