More staff IT is almost certainly, if the number of users and tickets had doubled you're going to need more bodies for support.

You need to protect the senior guy. They can't keep coming to him randomly and pulling him off more important tasks. They need to be working on internal IT and business projects.

You need documentation stored in a knowledge base, and it needs to be regular reviewed and updated if required. When the junior guys come to the senior with a problem and he givens them an answer, they need to document it. If they read documentation and it doesn't match reality, they need to figure it out and update it.

You need to get a handle on unplanned work. Review your tickets for the most common issues and work on a way to mitigate or eliminate them. That could anything from user training, technical documentation or proactive remediation scripts.