Sounds like he's doing wonderfully!
The MCHN told me they are not looking at bubs sitting unaided until they're around 9 months old. My DD sat before she crawled and I think that's why it took her longer to get herself around. Personally, I'd rather she have crawled before she sat, but I guess its not our decision, is it? LOL! Good on you, your son sounds like he's going great guns.