Assalamualaikum,
I have been married to a non muslim woman. I'm separated from her. Days before yesterday, she informed she gave birth a son but just after the delivery my son died. According to their custom they left my deceased son in a deserted place upon on the soil. I want to pray everyday for my deceased son so that his soul is accepted by Allah and he lives in heaven afterward. Please tell me what should I do for that. What duya shall I recite everyday for my deceased son. Allah hafez.

Inna lillahi wa inna ilaihi rajioon
Read Quran and pray for him.
God is great and works in mysterious ways which we humans are not even capable to understand.
He was a baby and did not commit any sin.
Inshalah he will be in heaven.
Pray for him