I would purchase a size 0. Only purchase a size 1 if the baby is larger than the average.
I'd agree with size 0 but you also need to take into consideration if he's a chunky fella now and if he's on the tall side... My daughter is tall but slim so depending on the clothes would depend if I went up a size or not. Err on the side of caution and buy the size up from what you think. For pants make sure they have an adjustable waist, they'll fit him for longer :0)