I am not a doctor, but I don't know what blood work to even consider. Anemia in a young child is SO unlikely and you would probably see other symptoms of that before you got to severe back pain. With a 3 year old, I would keep notes like a journal, of when the pain happened and brief context of the situation (for example, was he sitting for awhile, eating, etc.), have him rate it on a scale of 1-10, and how long it lasted. Bring the journal/date book to your next pediatric appointment. Engage your son in doing relaxing techniques to deal with his pain. He might be having a stress reaction to something. I think that if the pediatrician is interested in an ultrasound, they are certainly believing you and thinking about stuff. Unfortunately the way our health system works, the physicians are taught to rule out the most common things first and have to proceed on a step by step basis. As a mom, I know this is so incredibly frustrating. I agree with other comments that you should just keep on asking questions. With our son, we did find out over the years that things bother him more than other kids and we've learned to deal with that. There actually isn't anything wrong with him, he's just particularly sensitive to aches and pain from daily living and we deal with it accordingly. I see it now as a part of lifelong taking care of oneself. He's thirteen and we now are getting complaints of backache again. I don't know if it's backpackitis or growing pains or that he just started baseball and is using muscles he hasn't used in a year. But we take each case as it comes. (just one note: at one point with my son, we found out that one leg was just a tiny bit longer than the other and that could have made his back hurt when he was really little.) We went to a pediatric orthopod who said he'd out grow it and he did - thank goodness.
