I'm by no means an expert and I'm sure someone will come along with better answers than me, but I would think the frontline could have caused it. It is definitely worth at least a call to the vet to see what they say. I would be concerned that he could at the very least become dehydrated. How old is he? How much does he weigh. The littler ones are more at risk.
I would want to know what's causing this and nip it in the bud ASAP.
I wish you all the best and I hope it's something really simple.
Tammy