Repeatedly giving issues with "authentication servers being down" and timing out.

Hello, I've been hosting a modded server for Stoneblock 3 just for me and my bf. However recently he's been almost unable to play as he keeps getting kicked out. The issues are either him timing out, or "authentication servers" being down. His and my internet speeds are perfectly fine, it just seems like only minecraft is having issues. Does anyone have an idea of what to do?