I've seen this happen to many early-stage startups. Not exactly with a cofounder copying the idea, but along the lines of the idea being copied. My advice is the following:

Your biggest problem is still finding product/market fit. It's very possible that you both are still building a product will eventually fail. Or at least in it's current form.

Focus on getting your traction/revenue. Once you're raking in the cash, you can determine whether its worthwhile to go after your old co-founder.