IT is mucocele of lower lip. Which occur due to block age of salivary glands (minor)
Usually happen post trauma or due to infection.
If he is having this swelling since 8 months unlikely it will go on its own. And it needs to be excised.
Or if he is saying recurrent swelling and this swelling since few week u can wait & watch.
Next Steps
consult a pediatrician .
avoid spicy and outside food & bewarages.
plenty of oral fluids 2-3 lit /day.
fruits and vegetables should be involved.
Health Tips
ask your son to avoid lip chewing or nail biting if he is doing.
and proper chewing of the food.
and look for any dental infections.