Broken Capillaries

I'm concerned about broken capillaries/birthmarks in my 5 year old son.  They started to appear on his left arm a few years ago, here and there, which I wasn't too concerned about however they've now become larger and are appearing on his legs too.  
He does have what I've been told birthmarks but they were not there when he was born!!  His overall health is fine, he does complain about sore knees throughout the night, but our GP has advised that's it growing pain, he also suffers from the odd cold as well, which I'm suspecting is normal.
I have seen our GP about these capillaries and was asked to get a blood test but this was out of the question with my 5 year old.
Do I really need to enforce this blood test? Is it normal for him to have so many broken capillaries over his body? I myself have these broken capillaries and birthmarks throughout my body but not as many as he does!  What should I do?