So we went to the rescue centre today and fell in love with three babies - two girl and one boy. We've said we want all 3 and reserved them, on the condition the boy is neutered before we take him (the lady was very happy with this
How old are they? Your Boar-To-Be will need to be 4-6 months to undergo castration (but the weight and physical fitness is key . He might not be mature enough at 4 months) Then he needs to be kept apart till he's sterile (about 6 week) .
He will be fertile at about 4 weeks (so hopefully away from Mum )
You could keep him side-by-side until he's old enough and sterile (I have done this with my GP3 when he was left bereaved and I got him two adult sows . He was a year old) .
We got GP6 a couple of weeks post neuter so he was 4 weeks in waiting.
Then you need to devise a way of making sure he cannot escape into their cage to get up to mischief 
(Seriously though,) don't risk letting them breed especially if they are littermates , it can lead to all sorts of deformities for piglets .
The Rescue should have a vet that does a load of guinea-pig castrations . If they neuter hiim , you'd likely need to pay more for him. (Its about £55 to castrate , a Rescue will pay less most likely)
Red eyed white (REW) or Pink eyed whites are lovely ( I had a Himalayan who classed herself as Ruby Eyed she was quite light sensitive (and no matter what other owners say her eyesight was not as good as her dark eyed cagemates) . She loved being out in the grass at night time , sometimes 10pm (in secure rabbit runs) we were trying to catch them. "Time to go in , the foxes are about" she was a little toad "No, I'm fine" like a little phantom.
As a rough guide , three guinea pigs will need a floorspace of at least 5'x2' . Bigger is always better .
I had a wooden playhouse (DH adapted with secure mesh/bars in the windows that could be removed . We put 2 hutches in when we had 2 groups . They were on the floor with constant access to the floor . They had a divider that they could chat through. There was a light and a heater for cold weather plus a fan) .
The floor space was about 6'6" x 6'6" .
It was supposed to be big enough for 12 guinea-pigs gong by the calculations but even with my maximum pig herd of 5 ( boar+2 sows in one group and 2 sows in the other ) that was it full up !