How to boot the OS into single-user mode depends on if this is a SPARC or x86 based Sun server. Do you know the model name?
How to Use GRUB to get to Single-User Mode (Assumes x86)
Power-on the server from the LOM.
Get to the console (the LOM command 'console' should work)
You should see a GRUB screen asking which kernel to boot, leave it at the default, but press the 'e' key.
Look for a line like this (see below), add ' S' to the end of the line.
kernel /vmlinuz-2.6.18-274.18.1.el5.centos.plus ro root=/dev/vg/rot rhgb quiet
Press 'b' to boot, should go right to Single-User Mode (might prompt for the root password)
Example: See this blog post