Thanks for writing your query.
is an antibiotic which cause damage to beneficial bacteria present normally in our intestines. These bacterial flora help in digestion and absorption. Antibiotics cause loss of these flora and result in diarrhoea
Pain in leg can be due to loss of electrolytes due to diarrhoea.
You should give him probiotics to supplement the loss of normal flora of beneficial bacteria.
Also take care of his diet. He should have light diet.
Give him Oral Rehydration Solution if he seems to be dehydrated.
Hot fomentation will help in relieving some pain.
If the problem persists, get him examined by a Paeditrician.
I hope this is helpful to you.