My son has to have extensive fasting blood work (around 10 or 12 vials) is it dangerous to take so much blood at once? he's very frail.

Talk to your doctor: It is that he has to have 10-12 tests or has to have 10-12 tubes of blood drawn? What type of tests are being done? Most routine blood tests can be done on small amounts of blood. It would be prudent to talk to your doctor.

Extensive fasting blood work : First, that is a lot of vials, but shouldn't be a problem. Might want to increase hydration prior to lab by increasing fluids. How do you know it's 10-12 vials-rarely amount of vials are told until patient sitting in lab chair. Also , how old is son might determine any reaction. Usually, if concern for dizzy or weakness , most places will have patient laying down while drawing blood.