Son has one big toe nail that has fallen off & the other big toe nail is lifting. no trauma, fungus ruled out , nail flat/spoon, ideas of cause?

See below: Onycholysis is the separation of the nail plate from the nail bed. It can be caused by local problems like periungual warts or fungus (onychomycosis), but the most common reason for separation is trauma. However, some systemic diseases can cause this like psoriasis and hyperthyroidism. Have your pediatrician look into these possibilities.

I will provide an : Idea, it is a little difficult without seeing the actual foot. I do believe that fungus and trauma are the most common causes. Trauma does not have to mean a heavy object fell on the toe. Repetitive micro trauma like a toe hitting the end of a shoe when running etc could be a cause, and nails can be negative for fungus due to false negatives.