Thank you for your query.
I can understand you concern and appreciate your thinking for the correlation of pain with winter. I would have appreciated if you had shared the investigation findings for me to guide you. Have you tried eliminating particular food items from the diet that you use only in winters?
If your son has not been investigated then I suggest the following basic investigations to be done for us to come to some conclusion:
1. Stool routine
of whole abdomen
If the above investigations are normal and your son is gaining weight adequately then only will we think in terms of functional abdominal pain
I hope I have answered your query to your satisfaction.
Wishing your son all the best.