Theory on why John can keep constructs tangible without the ring

Total guess here, but his exposure to the ring as a baby may have had some effect on him that made him particularly gifted at this. Maybe that's why Lianna came to Bernadette in the first place? It's possible the whole "making amends" thing was bull shit from the jump, and Lianna made it up to gain access to a child exposed to the ring at such a young age.