J.K.
My kids are rarely sick. Per the doctor, I give them daily probiotics and at that age, 1,000 IU a day of Vitamin D3. The 400 IU that they recommend is WAY low. Almost all Americans are severely deficient in it. My 2 year old takes 2000 a day, and his blood test results for D3 are in the optimal range.
http://www.vitamindcouncil.org/
Also, a daily dose of fish oil is a good idea also.
And, yeah, I'd get a second opinion. Try a naturopathic doctor.