I'm copying and pasting relevant links directly from my thread tracker for Bolin's event/quest involvement history! In basic summary:
Mako is gifted an unhatched dragon egg to care for as a boon, and Korra is given a quest related to a dragon. She's told to research the fate of a dragon buried under Redgate: to find a cure for the poison slowly killing it, and to release it from its prison. Korra selects her crack team of investigators, Bolin included, who research in the library and journey to some old ruins hoping to crack the case! Meanwhile, Mako is really glad he's a firebender, because otherwise he'd have a lot of trouble with this egg.
Korra secures the dragon's spirit in a lantern, for when they find his body!
Bolin mostly had no idea about the precise cause of this huge battle that appeared before them, but Korra and Mako were going along for it, and he wasn't about to let them do it without him around. He engaged in some general battling and witty banter sort of stuff until that one time Korra ran past him with his half-dead brother.
They left the battle early to get Mako emergency medical attention. Medical attention that was successful! Thank. Goodness.
A warning for: blood, graphic description of injury, emergency medical stuff, tl;dr and sad feelings.
The Silmaril battle handled thusly, Korra's party heads out to Redgate (sans Mako, still recovering from his grievous wound). They meet with Alyosha and try to convince her to let them have access to Sura (the dragon imprisoned under Redgate's mountain), to no initial success. Powers are also bunked in the majority, bending included. For his part, Bolin mostly spends his time trying to get to know the people of Redgate, Alyosha included, because he doesn't see himself as a great first-choice politician in such a serious situation.
An Unseelie team also arrives, hoping to sway Alyosha to let them kill Sura where he lies. After some high tensions, a very explosive dinner, and a little bit of Unseelie underhandedness courtesy of Her Majesty, Alyosha forms a contract with the Unseelie team. They are allowed to find and kill Sura.
John Grimm, gifted with hella rad superhuman hearing, overhears the plan. Korra, John, Kaldur, Bolin and Naga immediately head out to the mountain hoping to beat the Unseelie to Sura. They succeed in arriving first, and Korra reunites Sura's spirit with his body. She spends a while kind of spirit-worlding it up with Sura and learning about him.
The Unseelie and Alyosha (who is really mad about this Seelie infringement) arrive on the scene. Hell breaks loose. People are fighting everywhere, there's fire, there's blood and death and violence, there are people climbing on the dragon????
Korra kicks Alyosha off of Sura's back after Alyosha lands a heavy blow to his eye. Bolin catches her, but it doesn't look good. He and John Grimm depart on Naga for an area not affected by the power cap hoping to heal and save her life. They find their path thanks to the fact that Sura kind of... tore his way out through the mountain, killing thousands of people, and taking off with questers still on his back...
John and Bolin save Alyosha, though. Redgate is very thankful, and Alyosha names John and Bolin official Champions of Redgate for their work. Hermione is kidnapped by Ganondorf, Cassie Sandsmark manages to escape but has to take the long, meandering road home. John stays behind to help Redgate however he can. Bolin takes Appa, Naga, Pabu, and three dogs back to the Seelie castle to regroup and rebound.
MEANWHILE, BACK AT CAER GLAEM: Mako hatches the heck out of that dragon egg. Rizhao is successfully born, into a lot of familial love and awe.
KORRA'S DRAGON QUEST, pt. 1
SILMARIL BATTLE
REDGATE. ALSO BABY DRAGONS.
L8R MOON, AND A SLICE OF L8R ALYOSHA
FALLEN WARRIORS AND INVESTIREDGATE
WHITE HART
BENDINGGATE, KORRA'S TRIAL
COPYPASTING: An Càrn Bàn: now it's not in capslock.
FROZEN PEOPLE: HOT DAMN