I agree that his boggart is probably a person and having something to do with the Potters. My opinion is that Severus's bogart is probably Lily being killed. He liked her when they were in school and probably fell in luve with her. But, since James and he were rivals and James wanted her, too, he never got the girl. And never got over her, either. Not to mention the fact that Severus had to hide his true feelings from his House mates. Slytherins didn't associate with Gryffindors for they were hated rivals. Severus was already having to cover the fact that he was actually a half-blood and not a pureblood. So Lily dying would probably be a good guess as to his boggart.
On to the Patronus. I have stated before on a totally different post that I believe that Severus saw Albus Dumbledore as a father figure. Meaning he was always closeted with Albus somewhere conversing about Voldemort and Severus's role as double agent. Fawkes, the phoenix, was probably present as well. So, here's a wild idea. What about Fawkes ? Could Fawkes be his patronus ? Severus trusted Albus, more thahn likely with his life, and vice versa. So, if Fawkes represented Albus, I would venture that as a guess to be his patronus. Fawkes.