My 3 years old son has white spot in his leg i wonder what s that im worried it will be i vitligo?

Fungus: It may be a fungal infection rather than vitiligo. Without a picture however, one can't give yiu a definitive diagnosis, so see your doctor for help.

Unusual place for it: Vitiligo is usually located on the hands, neck and facial structures and they are devoid of pigment, not just pale areas. Location as a single spot on a leg would be an odd location and more likely to represent one of dozens of other possibilities. I't bring it up at the next doc visit & have it looked at then.I wouldn't make a special trip for it.