I checked some of my favourite PEI resources (I grew up there, and it's where most of my research is currently focused). I didn't find anything pointing to a birthplace (except England, but nothing more specific than that). I did come upon these two things, which you may have already seen, but if not, could be helpful:
I have the book "Planters, Paupers and Pioneers: English Settlers in Atlantic Canada", which includes passenger lists. There is a listing for a "Mr. Riggs" who sailed from London to Charlottetown in May 1843. But given the above info, this is probably not your Riggs, since your Riggs was likely already in PEI by 1843.