You have to go out to options - controls and then look at Multicrew bindings.
You need to select a key to enter role, this will make you go from gunner in chair to gunner in space and back again.
You also need to go into right panel and set up bindings for weapons.
Another useful keybind is cycle UI, this changes the default view of ship hologram to radar view when you are back in gunner seat.
Took me about 15-20 mins to work all that out, and a post in the newcomers section to find out how you change the radar view.
The bindings can take a time, so be patient with other players that hop in, as first time nothing works.
Note as Helm you can disable SCB module and re-enable when gunner is happy with bindings, you don't want them firing off everything in one go!
Also consider push to talk button for Comms, so players don't hear you eating crisps all the time

Although if it's your son you will probably be shouting a each other across the room!
Fly safe!