I'm far from an expert, but I do recall when I first started shooting my bow that my arrows would occasionally fly to the right (I'm right-handed). A buddy of mine watched closely, and after a few fliers that went right he told me I was "collapsing" - relaxing my bow arm too soon.

I began mentally reminding myself to "hold through the shot", and cured my right fliers.

Since I'm right handed, and they went to the right, and your son is left handed and his are going left I thought this might help .... but like I said, I'm no expert.