Your son needs to make an appointment with the vet first thing in the morning. A few streaks of blood in a dog's vomit are not too much of a cause for alarm, but if there was any appreciable amount of blood in there, it could be a sign of serious illness and needs to be addressed immediately! Please post back and let us know what the vet says.
Ghilly
Take the dog to the vet as soon as you can. The dog could have swallowed something perhaps during the day, which may have caused damage either on the way down, or even in the stomach.