Thank you for your query.
The present complaint of back hurting can be due to the vaccination
which he received yesterday. Unfortunately you have not mentioned the vaccination which he has received.
The stomach issue can be due to some intestinal infection. It shall be better to get his stool examined to rule out a GI infection.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery.
Rajiv K Khandelwal