I read somewhere that he was more progressive reformed rethinking certain tenants of reformed theology, is that true? What was he trying to reform (no pun intended)?
His book on Paul (which I read) clearly redefines the doctrine of predestination. As a retired Reformed minister who learned under G. C. Berkouwer (who himself modified predestination doctrine) told me once: Ridderbos was doing to biblical theology what Berkouwer did to systematics.
In my humble opinion (based admittedly on his one, although important, book) he also modified baptism as well.