I would suggest working with a professional trainer on this problem. www.apdt.com may help you find someone near you. I'd look for a person who uses positive reinforcement training methods to help you in this situation.
It may be the boy smells different or just smells frightened.
For now I'd put the dog in a crate or another room with some special chew treat he does not normally get to have. Making the student's arrival into an 'oh boy he's here I get a XXX chew treat' can help limit the dogs negative reaction to the child.
The way positive training would work would be to have the student there and reinforce the dog's good behavior and working on your controls with the dog on down, stay, sit, quiet etc.
Hope this helps you!
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