My little bereaved guinea pig is really sad :(

[MadMommaMemoo]

His brother died last weekend

Put him in the run today and he just sat there, wouldn't even eat his apple.

What can I do to help him?

Sorry to hear about losing your guinea pig and your remaining piggy being so sad.

This happened to us too and we had a very unhappy guinea pig left, missing his friend. We solved it by finding another, similar aged guinea pig that needed rehoming. We introduced them gradually - our guinea that had been bereaved couldn't stop squeaking when he saw the 'new' guinea pig and followed him everywhere. After a period of familiarisation they lived together very happily for many years.

Guinea pigs do grieve. How old is he? Are you planning to keep him on his own now? Guinea pigs are really sociable creatures but I can understand that if you get another then the same may happen again and so on.
We have successfully introduced a new male guinea pig to a bereaved pig several times though. You have to take it slow and steady though, with the cages or hutches alongside each other for a while and then supervised playtimes together etc.
We currently have a single male pig and he sits alongside the cage the females are in and seems to be happy that way.
The alternative is to spend a lot of time cuddling and playing with him. Put him somewhere that the whole family spends a lot of time so people will stop and speak to him. Sit him on your lap (or the dc's laps) when you are watching tv. Things like that do help.
Is he well? Is there a risk that whatever took his brother could be ailing him? Check his teeth are even and that sort of thing.
Poor piggy.

Oh that's really lovely. We are picking up another one from a rescue centre next weekend. I just feel so sorry for him in the mean time. I do hope they get along!

We gave him a little pamper today, he loves having a bath and then being wrapped up in a towel and snuggled.

He is 4 and the one we are getting is 5 months but the rescuer thinks he will suite being with an older piggy because he's really timid.

Older pigs are more welcoming to baby piggies as well so it may actually help. Take it cautiously but as I say, we have done it several times and it has always worked out fine.
Keep pampering him until then and buy him his favourite foods and some Timothy Hay (or whatever his treat is) and he will hopefully bounce back.