Should I believe him?

Hello, everyone. I’m currently dating someone whose name is Michael, and we are close to becoming official. However, I am confused yet concerned if I will end up heartbroken.
Another boy whose name is Dylan messaged me on Facebook and told me I should be careful about my relationship because he said he saw Michael with his ex last week. Michael and his ex broke up in August last year but still hung out until April. Dylan knows both Michael and his ex. I questioned Michael about it and he said they did indeed meet up but for meetings/ it’s all work related (they work in the same field). He then said what are my concerns because technically he still is single. I said how can you say that? And he said he didn’t mean it that way and I’m the one he wants.
I dont know what to do or how to react. Any thoughts?
My ex left me in January for another girl. I dont want to be heartbroken again.