Every three days is good. They dont really get dirty and it will dry out their skin if you give him a bath every day. Once he gets a little older and starts crawling then you can switch to every other day. My kids are 3 and 1.5 and we give them baths during the winter every other day and wipe them down on the off nights.
NOTE: Giving baths every day wont hurt but it isnt necessary and washing their hair everyday can actually cause cradle cap since it dries their scalp more. The oils our scalp make and our body makes is actually good for us and for children as well.