pooing irregularly is perfectly normal in older BF babies (post 6 weeks), but if your DS has been like this since birth, then it could be that there is an underlying issue that needs looking into.
Giving additional fluids won't help though as BF babies don't really need anything else.
How is his weight gain?
From kellymom
early days "Dirty diapers: In the early days, baby typically has one dirty diaper for each day of life (1 on day one, 2 on day two...). After day 4, stools should be yellow and baby should have at least 3-4 stools daily that are the size of a US quarter (2.5 cm) or larger. Some babies stool every time they nurse, or even more often--this is normal, too. The normal stool of a breastfed baby is loose (soft to runny) and may be seedy or curdy"
weeks 4-6 "Dirty diapers: Expect 3-4+ stools daily that are the size of a US quarter (2.5 cm) or larger. Some babies stool every time they nurse, or even more often--this is normal, too. The normal stool of a breastfed baby is yellow and loose (soft to runny) and may be seedy or curdy. After 4 - 6 weeks, some babies stool less frequently, with stools as infrequent as one every 7-10 days. As long as baby is gaining well, this is normal"
and some further information here