Our current group is a diverse one. We have a very experienced DM (playing and DMing for ~30 years), 2 slightly less experienced players, who have experience DMing (~5 to 10 years) and therefore have a good understanding of the rules, 2 more players that have been playing for a year (myself included), and a greenhorn.
The balance is good. If one of us has a question, there is someone who will have the answer, even if the DM is busy. However, there is one member, of the 2 experienced players, that keeps playing the "assistant DM". The only problem is, his attitude is not always the greatest. When someone asks a question that seems to hold up the game (how to calculate a roll/what bonuses to add, do I get a skill back after a short rest/long rest etc.) he gets a bit "snappy".
At other times, we're trailing off a bit, having a good chat, getting side tracked, then someone wants to move the game forward suggestively by maybe re capping who's turn it was, or by asking for a description of the area etc, he doesn't always follow the flow - that is, if he's enjoying the conversation, he'll continue it, even when others want to get back to the game. Then on the other side of the coin, when he feels we're getting side tracked, he'll get snappy - "Come on guys, we're getting side tracked. Let's get back into the game!".
While his actions don't hinder the game (too much), his attitude does damage the mood a fair bit. How can we manage this/what can we do to overcome this?