Thanks for your query,
I think your son was bite by non poisonous snake.It will take 1-3 hours to get symptoms depends upon the intensity of venom penetrates in body.
Usually snake bite
causes bleeding disorder
so watch for any bleeding from mouth, ears, nose, in stool or in urine.
Watch for his mental status as he feels drowsy or falling sleepy etc.You can very well confirm this by asking some questions.
The last thing is give him booster dose of tetanus toxoid
.Wish him a speedy recovery.