I am sorry you and your child are going through this.
It is scarry for anyone of any age to cough up blood for the first time. I have done it several times and still get upset. However, my doctors don't really get that upset, so I am trying to learn to no freak out over it.
He should be seen by his asthma dr ASAP. As cargiver222 said, there is likely an infection. Since you already know he has asthma, it is most likely flaring and does need to be evaluated and treated - probably just an increase in breathing treatments, maybe an increase in ISC (preferred over a burst of pred), or even a pred burst.
Robitussin (and generics) make a guiafenesen product for children that I believe is for ages 2 and up. You would have to consult with the pharmacist, but I see no reason why you could not start your child on the expectorent before seeing/calling the dr. Guiafenesen (expectorant) is given to young children all the time.
I pray that he gets to feeling better soon and that you get some rest. I remember what I is like having sick little ones.
I feel terrible for the little one.
The blood vessels in the lung can be ruptured by excessive coughing, as well as infection or something serious.
A single instance should not be cause for concern, but I would notify his pediatrician.
The most common cause in an asthmatic is rupturing the blood vessels while trying to clean out the lungs of mucus. This is why an expectorant is helpful. It relieves strain.
I would not recommend an expectorant for a child without sonsulting his physician, but the chances are this will solve the problem.