The guy was driving a 14 year old car with no insurance and he can’t afford those repairs so he sees an opportunity to bilk someone into paying for them. You.
There is no way a comparatively minor rear end hit is going to cause any kind of blown gasket, fluid leaks in the engine compartment, or problems with the rack and pinion. A 1000 dollar estimate is barely a scratch when it comes to auto body repairs.
Your only responsibility is to cover the actual rear damage to his car; nothing more.
He threatens legal action. Big deal. You have not been served with any summons yet so do not worry about it at this point.
I can see him meeting with a lawyer now. When the lawyer hears you were not insured he will quite likely say sorry no can do. With no insurance involved the lawyer would likely want a very sizeable retainer to proceed with action against you and I doubt the guy is going to have that.
Lawyers like to bleed insurance companies; not people who are not insured unless they have very sizeable assets.
He can sue you but all he gets is a judgement. Enforcing that judgement is far more difficult than getting it.