I would watch and wait. I don't think a second dose would hurt him but it would be a waste of the product. Keep it on hand for future use.
I buy the probios treats for when we travel. It keeps the girls regular in a stressful situation so you may want to keep some on hand just for those extra stressful horsey days.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France