Hello Jill,
It is possible that he was a rigger and then moved to more responsible duties (although a rigger was a very responsible posting) an Admiral's Coxwain would be the peak of many sailors careers. It is also possible that in retirement he took on the post of rigger in much the same way as today one sees many retired ratings working in Dockyards. Since Britannia Royal Naval College is in Dartmouth it is possible that he was working there as a rigger and caught the eye of one of the senior officers thus helping him to become Admiral's Coxwain.
Rick