Please Please Help me!?

Theres nothing you can do but keep on searching for your lost piggy, get some cardboard boxes and cut out a guinea pig sized hole in them, leave them around the garden to try and entice him as he will be constantly looking for a safe place to hide. I really hope you find him. As for the remaining pig he is probably picking up on your distress of what happened as well as being upset about losing his cage mate so try to stay calm around him and give him lots of love. It will be useful to weigh him everyday aswell. Good luck.