What Causes Paleness, Headache And Leg Aches In Kid?

my gp has referred my 7yr old son for blood testsa and assessment. he dosent think it is anything untoward but i am worried as why send him to hospital if nothing serious. my son is very pale all the time with big black bags under his eyes, is tired alot and gets headaches and leg aches. his eyes have been tested and they are ok