The head coach and assistant are giving opposite instructions

The coaching team needs to align before asking the player to interpret two different messages. Both instructions may be useful. Staying wide could stretch the opposition, while coming inside might help a particular combination. The problem is that your son is now choosing between adults rather than reading the game. I would raise it privately and calmly with the head coach. Explain the effect you are seeing: he is checking the touchline before deciding. A good staff solution is to agree one lead voice on matchday and use shared language. The coaches can also make the instruction conditional, such as 'stay wide if the full-back is inside' rather than giving an absolute command. No coach needs to be blamed. The important outcome is that the adults become clearer so the player can return their attention to the pitch. Lead with the effect on your son rather than the tactical disagreement. You could say, 'I have noticed he is checking the touchline because he is hearing two different messages. Could the staff agree one cue for him?' That invites the coaches to solve the communication issue together without asking either person to admit they were wrong.