I am born 12/02/1998 and my boyfriend is born 08/15/1999, but his ex is born 10/28/1999.

I do not think it is needed to worry about it. The Chinese astrology is just for reference and not always ture. I think love is more real than the Chinese astrology.
If you have a good and steady relationship with your bf, please do not worry about it.
However, if your bf also keep contacting with his ex, you need to think about it carefully.

Thank you for the advice. Plus, he does keep in contact with his ex. Long story short, the ex kissed him multiple times while he's dating me. He got pissed and finally said they can't be friends anymore. A few weeks later she said she "needed a friend for help", and he's friends with her again. After he told me he was never going to say a word to her. We got in a fight recently and only broke up for two days, and the day we got back together she asked him out. -.- So yeah.

100% agreed with Ryan. Chinese astrology is just for reference. But on the side note, you seem to be in a unstable relationship at this moment. With that said, you are still in a VERY young age where relationships fluctuate and "love" comes and goes like wind.

My recommendation for you is to take it easy and treat him like a friend. This is my best advice. All the married couples I know met at work, through friends, in college and online. My previous boss married his high school sweetheart so.........the chances of marrying your high school bf.........slim. The odds are against you.