With regard to your tax liability for receipt of his gift/loan, I suggest you contact the IRS by phone, explain the basics, and ask the rules. I've found them very helpful in the past. Even if you were a family member, there's a maximum annual amount that you can give or receive another non-dependent family member as a gift--I think it's about $12k/year from any one individual family member. Otherwise tax liabilities kick in unless you structure the transfer as a formal loan.So should I file? And if I do, what will happen to him?
According to the facts you've described, if your ex-non-cohabiting boyfriend is trying to claim you as a dependent on his tax returns, that makes me leery that he would ever sue you. Doing so could also incur an IRS tax headache for him. Lawsuits become public knowledge, and mention of the IRS or taxes in a complaint or answer could prompt an audit or worse.
Maybe he's demanding repayment for tax purposes. If so, if it ever came to that, you could claim lax payment terms--interest at the current Fed Funds rate (i.e. basically zero) with an extended bullet (one-time) long-term repayment of principal after many years. But I'm neither a lawyer nor an accountant, so ...