baby is constipated 😥

My baby is 19 weeks. Just had his 4 month shot a week ago. He hasn't pooped for the last 4 days going on 5th. He has gone without before for max 3 days.he looks like he's really trying but no luck. I think he's really constipated. Dr said to give him water 3 times a day.is this because of the shots or is it my diet , oh yah he's exclusively breastfeeding. My poor baby is in discomfort, what can I do to help him. Please suggest