He's a bit of a loose cannon.

More English words

• He is a bit eccentric. He has slightly strange or unconventional habits.
• He's the life of the party. He is the most lively and entertaining person at a social gathering.
• He has a heart of gold. He has a heart of gold.
• How long is the layover? How long is the layover?
• Happy anniversary! Congratulations on your milestone!
• He’s very down-to-earth. He’s very down-to-earth.