Over the last week our 2.5 year old gerbil has really slowed down

Over the last week our 2.5 year old gerbil has really slowed down. We thought he was dying but a week later he’s still struggling. He will eat banana and drink small amounts of water. Tries to walk to us. Mostly just sits on his little den and rests. He’s lost weight and seems to be breathing heavier. I can see his breath or pulse in his eyes.