Putting white clothes on a little boy may be asking for trouble. They tend to get dirty (both the clothes and the boy, that is). But they do look great, and for certain formal occasions, they are the most appropriate. You will have to chose whether you want a short-sleeved shirt and short pants, or long sleeves and long pants. You can go for a white tie, or go for a contrasting look with a black neck or bow tie.
Two examples I think are quite nice:
Boys white linen suits pictured:
Left: Gino Giovanni Baby and Toddler Boy Summer Cotton/Linen Blend Suit Vest Short Set
Middle and Right: Johnnie Lene Boys Cotton/Linen White Summer Suit From Baby to Teen