Searching for Robert Cann

Robert Cann was my paternal Great Grandfather and has been my most persistent brick wall since I began my research at least 15 years ago. Family tradition had him born in Tiverton, Devon, England on the 28th of May, 1827. I have not been able to find any information on his parents or siblings. However Canadian Census reports indicate that he came to Canada in 1858 and was born in 1832 (1871 Census) or 1833 (1881 Census) or 1831 (1891 Census). It is not until the 1901 Census that he is listed as being born in 1828 and then the family somehow ended up with 1827. A search of the 1851 England Census brings up a very interesting result in that there is a Robert Cann listed as an inmate in some sort of institution in Crediton. What makes this particularly interesting to me is that he is listed together with a younger boy Thomas Cann age 11 and the family has mentioned they thought Robert had a younger brother Thomas but they had no information on him. Robert is 18 in the 1851 Census which would mean he was born in about 1833. Looking back to the 1841 Census there is also a Robert Cann, 8 years old, born in Devon and living in Crediton with his parents John and Mary and an older sister Sarah. My Great Grandfather named one of his daughters Sarah and another Mary. Could this be the same Robert? And if so, what happened to his parents? Where are Sarah and Thomas and how do I tie this all together? Canadian passenger lists don't start until 1865 so have been no help. If anyone can help me piece this all together I would be most grateful! Thanks.