My son is 10months old and his gum is bluish purple what can this be ?

What is it?: We can guess over the internet but that won't help your ten month old. Have him seen by a Pediatric Dentist (Pedodontist) asap for an evaluation.

Teething: Most probable cause is baby teeth trying to erupt. Best to have him seen by Dentist, preferably a Childrens' Dental Specialist, a Pedodontist. All children should have their 1st dental examination by Age 1. If cause is teething, no treatment will be necessary.

Eruption cysts,: also known as eruption hematomas, are a dental condition that affects children as their teeth grow in. These cysts can form before your child's deciduous (baby) teeth erupt or before their permanent (adult) teeth erupt.