We really do adore Boq. If only he and Elphaba had continued hanging out in middle age. It would have been entertaining and probably beneficial for Elphaba.
In a lot of ways, Boq is Elphaba's best friend, and a better friend than Glinda. Their relationship is very genuine. Boq joins Elphaba on her crucial quest to discover Animal origins. His role in this enterprise cements their friendship. These two characters have a genuine affection for each other, shared interests, and the ability to be honest with one another.
So this is arguably the best sketched friendship in the book. Part of this may stem from the fact that we get their friendship from Boq's point of view. When we have Galinda's point of view as a focal point, she and Elphaba aren't really great friends yet. We also get a hint that the summer that solidified Boq and Elphaba's friendship would be an important one in Boq's life:
In years to come – and Boq would live a long life – he would remember the rest of the summer as scented with the must of old books, when ancient script swam before his eyes. (188.8.131.52)
It's interesting that Boq's long life is emphasized in this style shift, where we break away from a focus on present events and jump forward to a distant future. Boq is the only character who fully grows into maturity. We get to see Boq learning and growing over time, following him from his superficial crush on Glinda to his words of wisdom for the adult Elphaba.
Through Boq, Elphaba has a tangible connection between the different eras of her life – from her awkward Munchkinland childhood, to her Shiz years, to her return to Munchkinland as a Witch. Boq anchors the narrative for us, and Elphaba's complex past for her. He's both an element of continuity and one of change, since he actually ages and demonstrates that aging in the narrative.