sores (Aphthous Ulcers
) can closely resemble Ulcers caused by Viruses like Herpes
Canker sores are not contagious where as viral one is contagious.
Usually viral ones are associated with fever
and not canker sores.
Since your son has also got the same, its likely that both of them have virus.
So its better to have your son treated in the same line as your daughter.
You may use some local gels like Zytee to ease the pain.
Give him bland- non spicy food and fluids like milk, porridge, fruit juices, soups etc.
Hope my answer helps you.
Wish your kids a speedy recovery.