Can’t get blood from my baby?

Hi,

My son just turned 4 months today. He has been running a low grade fever for a month now. They have done a numerous amount of tests on him so far but the last thing that needs to be done is blood. My doctor had sent me to lab Corp, there they poked him 4 times to no avail. So his doctor told me to take him to the children’s outpatient center, there they poked him 3 times and again to no avail. What can I do or get the doctors to do to get my sons blood? Has anybody been through this and ended up finally getting blood. Thanks.