A teammates on the line, not playing the current point is he considered player or non-player?
They are a non-player (see the definition of player).
Can he make calls (for example call “out-of-bounds”)? Does it make any difference if he’s the captain of the team?
No. Only a player can make a call, regardless of if they are a captain.
Sometimes people on the sideline use hand signals to show if they think someone is in/out of bounds. There is no specific rule against this, however it may depend on other circumstances whether this is good spirit or not.
If a player makes a call, they should be doing so based on something they believe has occurred. They can then ask for advise from the sideline to help them make the appropriate call.
Hope that helps.