My ex decided he wants to come back after breaking up with me?

Tell him thanks but no thanks. You don't trust his motives, he's not making a very good case for being trustworthy by wanting to keep the relationship a secret, and things will just collapse all over again if you both can't (or aren't willing to) find a way to resolve the problem with your parents (or his mental health, if he's still not being responsible about treatment). I know you care about him, but sometimes that's just not enough. Block his number or find a way to distract yourself so that you're not tempted to pick up his calls, and keep trying to move on.