I'd say no
You can surely do better
Sounds safe in a non exciting way
Don't waste your time, there are better men to give your time and energy to. Let some drama queen have all that drama and baggage.
In that case, yes, you should respond to be nice to an old friend. I thought you were contemplating dating this guy, but since you are already in a relationship, then responding would actually be a nice thing to do for the guy.
Just make sure he knows that you are taken.
Then its all good.