My ex just asked me to go out for ice cream later... I'm not sure what to do?

We're on good terms right now, but honestly knowing him if I say yes he's going to take it the wrong way and think it's an okay to date again which it isn't. Also knowing him, even though we're not together he probably bought be a ridiculously expensive present like he usually does and I'm not comfortable or okay with that. The problem is that I don't know how to say no without setting him off or making him mad and when he's mad he's terrifying. I don't need to deal with the stress or him going around being an ass over this.