Lily is still working on sitting up. She can do it unassisted for several seconds but once she starts to lean she topples over.
Amiah has been sitting since 6.5 months and crawling too. Her balance I'd say is pretty good. She pulls herself up to a stand on everything, including my coffee table (and has chewed little bites into it), the couches, my pants, the window....silly baby. She also walks with a little push toy, but mostly goes to fast for herself and falls down. She was an early Sept. baby (4th) so a bit older than your Fynn. Those few weeks makes a big difference.
Kylie was like that for awhile. She started sitting at 4 months unassisted, but for a long time used her hands to balance her. She also had to lean forward a lot to maintain balance. If she lifted them she toppled. Now at 7 months she can sit without her hands or anything, and can also catch herself from falling over. She was an early sitter though, so don't be concerned at all about his progress.
As for standing, she doesn't really like to stand. She curls her legs and stands hunched over. She also keeps her feet together and makes a V looking thing with her legs and feet.