It's possible to be allergic to soy, I don't think vitasoy is good for babies though, doesn't it have sugar and other stuff? You can give sugar free natural soy milk again in a few days and see if the symptoms return.
Is it only on the leg? A bit odd for food allergy rash to only be there.
Its a possibility. Perhaps you cn monitor his subsequent intake.
Best to consult the pediatrician asap...
observe for three days
Ha