I agree Nrh. A lot is going on inside his gut and with his teeth right now so he might be runny off and on for a while. Totally normal at weaning time for stress to add a little cow pie too. He is developing his normal bacterial flora too so a probiotic could help get things more balanced for him. There are massive changes in the intestinal epithelium going on when they switch from a mostly milk diet to grass and hay.
I would say to keep an eye on him, and check his temperature regularly. If he starts acting depressed or spikes a fever, or if the diarrhea turns worse or is consistent for several days then I would think it might be time to call a vet. Being a foal is tough business!