The five lights that surround it are called Prominence (プロミネンス,"Purominensu"?).
The Son of the Sun will counter attack with Flare if he is attacked by anything. The Son of the Sun cannot be damaged directly. To defeat him, the player must attack one of the five Prominences. If the correct one is hit, it will cause the Son of the Sun to take around 213 damage. If the wrong Prominence is attacked, it will counter attack with a weak fire attack. The Son of the Sun himself will attack with Flare and a powerful magical based beam attack. It is recommended to wear Fire Protection such as the Red Mail, Red Vest, or Ruby Vest in this battle. Without it, characters can easily take 700+ damage from Flare, and the beam attack can do around the same amount to a single target if their magic defense is lacking. Magus's Barrier spell is very useful against this, as well as Barrier Spheres. It'll take about 10-12 correct hits to defeat him in total. It's best to use red mail on Crono and attack with Cyclone but this strategy will take a while.
If Magus is on your team, Black Hole can swallow up 2-3 of the boss's fireballs, making it much easier to attack and find the right one
Using the Red Vest or Red Plate makes this battle much easier. In fact, they can make it impossible to lose. You can obtain a Red Vest and Red Mail in Guardia Castle in 600 AD and 1000 AD, and Ayla can charm a second Red Mail off of Rust Tyranno in Giant's Claw in 600 AD.