I am sorry to hear of your loss.
Your boy will need a new friend and he certainly isn't un-bondable. Piggies are social creatives and although he will miss his brother he will also hopefully continue to live a full life and should not do so without a friend.
You have a couple options, you can find him a new friend either by a rescue or by a pet store.
We would recommend you go through a rescue as there are many piggies, old and young, looking for homes, they may also be able to offer you "boar dating" service which consists of you and your boy going in and meeting a boar or two and seeing how they get along, meaning you do not take a pig home and then have to put him back because they do not get along.
Having a bond is not fully down to being siblings, there was always a chance your boys could have fallen out as it all comes down to personality, which is why there is no guarantee your boy will get on with a pig you take home.
You can also "foster" a pig, so perhaps if you get a younger boy from a rescue and he gets along well with your boy, when your boy eventually passes the rescue will take back that boar and again find him a suitable home.
If you decide to go via a baby from a pet store then this is also okay. All introductions must take place in a neutral area with nothing but a big pile of hay in the middle.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
You can also rehome him, we would recommend you do this to a rescue who will find him a suitable partner/new friend and then to a suitable home. I would not recommend going through places such as pets at home as they will not monitor where you boy goes to and I can imagine you would like him to go to a nice home if possible.
As for grieving, a guinea pigs grieving after loss generally lasts around 4 days until the ultimate need for survival will kick in. If your boy is eating, drinking and acting himself then this is a great sign, but does not mean he should be alone.
It sounds as if you have a tough decision ahead, and I wish you well with whatever way you chose to go around this.
A Comprehensive Guide to Guinea Pig Boars
Death, Dying, Terminal Illness; Human Grieving and Bereaved Companions: Information and Support for Owners and Their Children
Single Guinea Pigs - Challenges and Responsibilities
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