You need to talk to a lawyer before any interaction with your company. Anything you say to their HR, your former boss, etc. can and will be used against you, should the situation escalate legally. If you email your former HR you may make exactly the slip of tongue needed to take your old boss entirely off the hook.
Consulting with a lawyer is safe. You do not need to sue or serve them papers of any kind and lawyers do not just randomly do this without your asking them to. If your lawyer is pushy then find a better one.
Your city or country may offer other legal resources but I'm not an expert here. Maybe someone can post an answer filling out the kind of regional resources usually available.
You have the option of just letting this go as any action you take will have consequences on your career. Even consulting with a lawyer costs money. Maybe they won't be bad consequences but you still should not be reckless.
I'm not sure whether there is any material upside for you. If a lawyer wants to sue for damages treat that as a doctor telling you you need surgery: it's a serious operation, with lots of risk, can drag on and on and be costly, and you probably want a second opinion. If you just want justice then you may have better options, but you are of course solidifying your former boss as an enemy instead of merely as a bully who doesn't otherwise care about you that much.
I understand that justice may be important to you and I'm glad some people pursue it, even when it can probably only hurt them. In this case, consulting with a lawyer or regional resources is step #1. That's your decision.