It is great that you are thinking rescue, but need to consider a couple of things:
Guinea pigs should not live alone, and most reputable rescues will not rehome a guinea pig into a single situation. They will also want to know about adequate winter protection and plenty of space, even more so for young guinea pigs.
Boars can be neutered from 4-6 months onwards (when depends on the operating vet, who makes his decision based on development, weight and general health). A boar will then face another 4-6 week wait until he is safe to join any females (100% safety means a full 6 weeks wait). You will need to find a good and experienced vet because there is a not inconsiderable risk of complications in the weeks after an operation, and finding the right vet is crucial to the success rate.
