Boku (ぼ く 、 僕). It is exclusively used by men. It has a strong connotation of humility and servitude.
Ore (お れ 、 俺). It is exclusively used by men. It is informal and although it has a slightly edge connotation it is normally used among friends, family, etc.
We could deduce that Ichi no longer pretends or doesn't want to show himself as his friend (ore), perhaps the author with (boku) wants to tell us that it is his way of moving forward in the relationship, I don't know...