If he can see the traffic relies on the encryption. If you visit webpages delivered by
http he is able to monitor everything. If you use
https the traffic is encrypted. In a company it's anyhow possible to decrypt the traffic if the admin installed a fake CA which private key he owns. There are various tools that implement such a proxy functionality like
MITMproxyif oyu just want to try it on your own. You can verify this by compüaring the OS/Browser CA-List with the one deliverd by the OS/Browser vendor.
Even if you use
https and the admin isn't able to decrypt the traffic he is still able to find out which domains you connected to because you are using the company owned DNS-Server.
You can use TOR to hide both but there are some downsides. The usage of TOR can be detected by the network administrator and this makes you or at least your PC suspicious because TOR is often used by malware.
You can use a VPN which is also detectable but not as suspicious as using TOR.
The best solution for your problem might be using your personal PC/Laptop at home.