Yes, there's a 4 year SOL. That is only for overcharges. There is no SOL for RS status. Figure out how much the overcharge might be before deciding.
Look at the RGB rules for vacancy leases to see how much they could have raised the rent for each vacancy or renewal going back. Look at the vacancy increase (usually 18-20%), the longevity increase and any IAI increases he might allege. Of course without registrations I would argue he's waived any right to come up with an explanation.
If he's screwing you, he would have done it to previous tenants. Look for patterns of fraud.