M.P.
It's easy to think that toddlers don't get cavities or that cavities are unimportant but that's just not true. That is why dentists recommend taking a baby to see the dentist soon after their first birthday or when they have many of their teeth in place. An abscess is usually caused by a cavity.
A toddler is often not able to adequately brush their teeth or floss correctly. That means an adult needs to follow up the toddler's efforts every day. Combined with this is the fact that some baby teeth have weak enamel which makes it even more important for an adult to also brush and floss the teeth.
It's important to brush at least twice a day and floss at least once, to avoid soda and other sugary drinks, and to have a nutritious diet high in calcium, vit D and magnesium. If he's unable to have dairy products then he needs to have supplements.
My grandson saw the dentist, his mother brushed and flossed his teeth 2-3 times/week. He drank milk, ate lots of cheese and took a multi-vit with minerals but yet he had several cavities, which in 2 teeth necessitated crowns at the age of 5. This happened in a 2-3 year period during which he didn't go to the dentist. He is a grazer when it comes to food and so usually had some food in his mouth. He didn't get candy at his mothers.
He spends 2 days/week with his father who gave him candy and soft drinks and didn't brush his teeth, thinking it wasn't necessary. This is a common misperception.
Do take him to a pediatric dentist and follow up as suggested even if you don't have dental insurance. Cavities in baby teeth can result in misshapen and out of place adult teeth. An abscess, if left untreated can not only destroy the permanent tooth but also continue to cause a serious and life threatening illness. It's good you saw the abscess.