Actually if you take Lenamare at his word he was originally planning to show his students how to bind a bound minor demon when he found Tom. He thought that with his talent he could easily bind an unbound minor demon. He was quite surprised as he had believed most of the demons already bound so new demons were quite rare.
Tom then started to struggle more as they initially reacted thus bringing up his resistance, but Lenamare was ill prepared to bind a demon. The fact that Tom comes from a magic poor world is likely the only reason he was bound as people from a world rich in magic would more strongly resist speaking their true name as Tom was at the time not quite as suspicious as he should have been. Tom could in fact be stronger than a type IV and if so it is quite likely he will grow into that role quite soon. He is already a powerful IV so it is feasible as he has yet to learn.
Not quite sure what happens though. I know demons exist on a spectrum of power, but I lack knowledge as to what occurs when they advance. I would assume that a demon that was originally red that advanced to yellow would appear different than it had. I am also quite sure that a demon bound with a spell of a lower rank than it was would grow faster than a demon in the reverse situation, but for clarification ask Tizzy as my knowledge of Astlanian demons is lacking in some areas.