My son is almost 11 years old, he is suffering from nose bleeds. He can have up to 3 in one day, sometimes he gets them when he gets upset, please can you help, can it be something serious?

Could be caused by allergies, the air in your house being too dry, blowing your nose too much and too hard or if he's a nose digger :). If he gets them when he's upset it can be the stress and blood pressure going up causing the capillaries in his nose to burst and bleed, especially if his nose is dry.
My mother and I get frequent nosebleeds caused by a mixture of chronic sinusitus, allergies, and dry air. I have to sleep with a humidifier on, but a lot of times when I get stressed it causes a nosebleed.
Could also be one of the capillaries not fully healing and each time he blows his nose it breaks open again. They say you shouldn't blow your nose for about 24 hours after a nosebleed, but for someone with a stuffy nose that's pretty impossible.

He might be low in Vitamin K.