what is the weight of your baby?
infants at this has pot belly and it is normal. as per you he is little skinny, that may be the cause that his abdomen may look distended (due to than abdominal wall/ normal at this age).
acording to you he is active. so do not worry if he is active feeding properly. if he is underweight
seek advise of a pediatrician
and he may need dietary supplement.