Are the eyes turning outwards i.e. away from each other. If thats so, then this is something that is not related at all to the trauma. It is a type of squint
that we call intermittent exotropia
. This will manifest mostly when he is either very tired, not paying attention, or is generally sick. there are some exercises that can be done to improve the same. Surgery for the same may also be needed but it depends upon how much he is squinting.
Hope this helps