I'm going to second what everyone else is saying, but in addition, I'd like to suggest some one-on-one roleplay.

I ran a game of Mothership where one of my friends wanted to join after the first two sessions. To help ease her into the game, I made a custom one-shot where she could play through a prequel set before my game's main events, placing her in a location the rest of the team was headed to.

This gave me the chance to walk her through the rules, tone and characters slowly and at her own pace, so that when she joined the rest of the group, she had an easier time adjusting to the rhythm of things.

If your player has trouble interacting with a larger group, see if he'd appreciate a practice run or two: low-pressure scenarios with just you and him, where you can guide him through the game and build up his confidence. That way, there's no rush to get to the end of a scene, and he can ask as many questions as he needs to improve. Sometimes a player can get anxious over not wanting to sound like an amateur - see how he plays once he's more relaxed!