Its okay to give probiotics with Amoxycillin
. But the recommended dosage and frequency of administration should not be exceeded.
Pain in his anal region- If he has had any difficulty in passing stools or had passed hard stools before this- chances are there for a fissure to happen- which will be extremely painful. This can be found out on examination and will need local gels and stool softeners for about 2 to 3 weeks duration.
If he has had loose stools before this- with constant wiping and constant contact with stools- chances are there that he might have developed peri-anal skin excoriations. In such a case it might hurt him lots even when he sits. For this he might need local ointments
like siloderm with a simple antibiotic cream as needed.
Probiotics and yoghurt- have many health benefits- and will be more useful in cases of viral diarrhea
Hope I have answered your query.
Wish your grandson good health.