Awww that's so sad. When I got my first guinea pig I got her a friend (who turned out to be a male afterwards) and they were thick as thieves. They never left each others sides and if one was brought out to be held the other would wheek and sulk until either they got taken out or the other one was put back. My female (the first one I got) died very suddenly and all I know is that she had diarhea. It could of been some sickness, but I also wonder after I found out that Rikku was male, if he could have gotten her pregnant and her body just couldn't take it because she was very young. I don't think he ever got over her dying. You could see it physically in him and how he acted that he mourned her for probably a good month before he started to perk up again.
However guinea pigs never fail to surprise the heck out of me so maybe he'll be fine in no time and if he's exploring happilly when he's out of his cage he may just be bored without someone in the cage to make it more interesting to play hide and seek and chase.
