Paul Walker’s dad says Roger Rodas’ family is holding $1.8 mil worth of his son’s exotic car collection … and he’s demanding the cars or cash immediately.

Paul Walker III filed legal docs claiming the estate of Rodas — who was driving the Porsche Carrera GT when he and Walker died — is in possession of numerous vehicles belonging to his son.

In the docs, he says the cars were all fully or partially owned by Paul. Of course, Roger and Paul co-owned Always Evolving … a performance and racing shop.

For what it’s worth, Rodas’ family already rejected the claim — so it looks like a judge will have to sort this one out.