The doctor may have ordered a blood test to make sure the child's kidneys are functioning properly. Also, see:
Nocturia (Nighttime Urination): Is It Nighttime Incontinence?
http://www.healthcentral.com/incontinence/c/45/12997/nocturia
Childhood Enuresis (Bedwetting)
http://www.healthcentral.com/incontinence/c/45/1799/enuresis
How to Discuss Enuresis (Bedwetting) with a Child
http://www.healthcentral.com/incontinence/c/3328/3568/child-part-1
I hope this is helpful. Good luck. - MG