Blood tests are very possible and can be done at any local physicians office. Hell, your pediatrician couldve drug screened your blood.

Urine tests have many different ways around them from pills to drinks to fake piss but blood tests you wont be able to fake.

And im pretty sure that cannabis can be detected in the blood up to 3 months, especially if he smokes everyday.... but im not totally sure tho..

I DO know that THC comes out of your urine for up to 2-3 months if you smoke everyday...meaning... if you smoke everyday, and you quit tommorow, you can easily fail a urine test for up to 3 months.