I am so sorry for your loss. How old is he? Sometimes when pigs are up in age owners keep them by themselves if their buddies pass away. You could consider getting him a buddy, which is what I would do. Guinea pigs are not solitary animals and they tend to do best when in a group of at least two or more. You mentioned "buying" another pig. Please keep in mind that their are many pigs near you in rescues who need homes. If you purchase a pig, you wont really have the opportunity to go through the introduction process like most rescues and adoption centers offer. And an introduction process is something that really needs to be done. I would say to look into adopting another pig. But please look into adoption before purchasing another pig.
Pigs, Just like people, all have their own personalities. That being said, each guinea pig acts a bit different when different situations occur. It isn't considered "abnormal" for your pig to not show noticeable signs of mourning, but nor would it be abnormal if it did. Just give him lots of love no matter what you do.
best of luck to you.