....

Miki was frivolous and didn't care about his grades or the future, so, his mom and Udou tricked him into getting serious and studying by implying that he didn't care about the relationship, that his feelings were not enough and it wasn't going to last. After that, he does study, even dyed his hair (dying your hair in Japan is kind of considered kinda rebellious and inappropriate) and his boyfriend reveales the plan at the end, he then ask if his brother was in on the plan but he wasn't, so finally at the end the brother has to apologize and Miki explaines that even if he didn't have a plan, he still never thought of a future without Udou.

If this was not the ending you were referring to, I will gladly explain the others! Also, if you want to know why the brother was mean and who he was at the end, here is his story: http://www.mangago.me/read-manga/owaranai_fukou_ni_tsuite_no_hanashi/