My little guy was born about the same time and he is 30" long and weighs 380g. If yours is any where near that size I would get a medium size. If it seems too big, just put some balls of newspaper or paper towels in it to take up some space, and that size will last them for a while.
I use a "large" for my adult and he is 54" long and weighs 2200g. I am amazed at how he gets himself in there, but he does. I even tried switching to a jumbo rock hide and he didn't use them. So once I put the RBI hides back in it was clear what his preference was...


Rob