Not wanting to breed is excellent, but most boars can get along with other guinea pigs.
Cavies are herd animals and most love having a buddy. How old is your pig? He's obviously over a year old since you've had him since January 2005.
Check animal shelters and rescues for a young (under 6 month) boar or a spayed female. Either would be a good friend for your guy. Alternately, you could check with your vet about getting him neutered, and you could get any female piggy. It's probably best not to use the plexiglass depending on how high up you have it. There is no ventilation through a solid material like that. Check the directions when you click Home at the top of the page, and you can make him a huge cage to play in pretty cheaply. There is also a chart with the recommended sizes for multiple pigs, so you might want to build a bit larger than you were thinking to have room for a buddy when you find one.