Hi and welcome!
Which country are you in and how old is your boy? Hove you had him for long? Your options depend largely on the climate and ideally on access to a good rescue, too. Please click on your name on the top bar, then go to personal details and scroll down to location, so we can always give you advice and recommendations that are appropriate for your part of the world. We have members from all over the globe on this forum.
Many of us are keeping our piggies indoors with access to the lawn in summer. I am one of them. If you are in Britain like me, please make sure that you keep the heat down as much as possible at the moment and make sure that the run provides plenty of shade. In winter, you would need to provide extra insulation or bring your guinea pig inside anyway.
What you can improve is company of the same species for round the clock stimulation and improved wellbeing/health. Guinea pigs are group animals and not wired to be on their own. Depending on his age and any access to a good standard rescue or a piggy savvy vet with experience in neutering, you can consider to let him choose a mate at one of our recommended rescues under expert supervision at a good rescue. This minimises all risks for you and him as you will only come home with a new friend if there has been success.
Neutering is an option if you would rather consider a sow as a companion in the long term; unlike with rabbits, it doesn't change boar behaviour. In that case, you will have to factor in a full 6 weeks post-op wait after the op and the need to find the right vet to minimise any risks of complications. Cross gender bondings (once initial acceptance has happened, ideally at a rescue again) are the most stable.
Please take your time to research your options, including any changes to housing. But you will find that two guinea pigs will provide you with double the fun as it is their lively, vocal interaction that makes them so unique and endearing!
http://www.theguineapigforum.co.uk/threads/faq-companionship.37654/
http://www.theguineapigforum.co.uk/threads/boars-sows-or-mixed-pairs-babies-or-adults.108944/