Welcome to the forum Meg, I am sorry to hear you lost one of your guineas.
How old is your lone guinea, and what size cage is he in? Often piggies are more amenable to new friends when they are grieving, so he may surprise you by accepting a new friend without as much fuss as you expect.
I have to say it sounds like he is a very dominant character, a rescue may be able to help you pair him up with a more submissive character, or even allow him to 'choose' his own friend.
If he will not accept a new guinea friend, then it's ok to have him living alone as long as you give him plenty of attention and keep his cage close by the other boars cages; ideally have a divided cage so he can interact with them without actually being able to get to them. This way he is not totally alone. For some pigs, only one pig will ever do and pairing them up with any other pig isn't easy, but if you have exhausted options of pairing him up then it may be that you have to just have him living nearby other pigs rather than in direct contact with them.