Events might have moved on since you posted this, but I had a look in the transcription of the Bongate register on a recent visit to the records office, but the only Robert I could find was baptised 13 Nov 1827, son of Robert & Dorothy.
So it seems he was either baptised elsewhere or maybe at a later date. Could they have been non-conformists? or could Robert & Dorothy have died and he went to live with John & Mary. Have you found the baptisms for any other children?