my baby is 2.5 months old , he is completely on breast milk , from last 3 days he has not gone potty , he is passing lot of gas. anything to worry about?

exclusively breastfed babies some times don't pass stool daily, even up to a week. there is nothing to worry. your baby should be feeding regularly and be content.

how many days should I wait for him to do potty before I see a doc ....

as I said if he is doing well ie he is active and Breastfeeding at regular intervals, has 8-10 wet nappies at the end of the day, you don't need to worry.

If your baby is in discomfort for eg trying to pass motion and unable to pass, seems constipated, only then visit the dr.

not at all, sometimes baby poo after 4-5 days also, passing gas is normal