Should I accept his request?

I’ve been dating my boyfriend for 3 months now and we are extremely happy with each other. today my ex added me on snapchat. it’s been about a year since we broke up. we didn’t break up on horrible terms, but we also didn’t break up on great terms, we never talked once since our breakup and that was it. I’m really happy with my bf and I don’t want to possibly mess anything up. Should I add him back? or just decline it ? .. he is also in a relationship now too.