Is Rika younger than Ren? I forget their ages. This is good stuff!
Rika is 14 and Ren is 15, but they're both in the 9th grade. Since that means that they're definitely less than a year apart, and since Rika's birthday is probably September 28 (judging from her personal memoria), my guess is that Ren's birthday is sometime between April - July. My personal guess would be May since the month is associated with the lily-of-the-valley, though I'm unsure if the whole birth flower thing is applicable to Japanese culture.