Basically, you can do either.
I would prefer to have them living side by side first, so they can get to know each other. You could then try to see when one of the girls is rather interested in Fred and stays close to him - she'll be coming into season and you can time your introduction accordingly. Choose somwhere with lots of space!
The intro will be livelier that way, but success is pretty much guaranteed. Just brace yourself that a boy who encounters females for the first time is in all likelihood going plain bonkers with testosterone overload. You'll be amazed at how much stinky testosterone one little boy can produce (a lot is in the wee - and he sprays it liberally EVERYWHERE, especially on the girls). He also goes humping pretty much constantly (unless he's crashing out with exhaustion), but is not fussy which end of girl he can get hold of... After a day or two in most cases, he will find the "on switch" for his brain again, and it will never be as bad as this again.
Make sure that you only have hides with two exits, as a lot of piggy courtship consists of rumbling (in context with a girl: "look what shmashing example of manhood I am and how much I want you" ) and chasing. The girls will be squeaking A LOT. This is NORMAL and the girls CAN handle a male going mad! You will hear a lot of regular rumbling from a boy with a harem; it's the way a boy woos his girls and is NOT part of a dominance struggle for hierarchy.
If a girl is in season/is coming into season, she will stick close to her man and will make a special squeaking pattern that sounds like she is being terribly put upon. It is to say "I'm not ready - YET - but I will be soon". It seems to have the effect of spurring the boy on. He also is liberally spraying the girls with his testosterone-laden pee in order to mark them as his. The equivalent of a wedding ring is a yellow patch on the nose! At the end of a girl being in season, she is finally ready to mate with him for about a couple of hours and will lay down for him.
If girls don't want a boy, they kick at him and spray pee back at him to literally tell him to p*ss off. Be aware that it is the girls who call the shots!
If girls physically go for a boy and chase him off aggressively right at the beginning, you have to cancel the bonding - it won't work most likely; anything else you just have to sit through even if it looks a lot rougher to us than it is for guinea pigs; bear that in mind - they are wired for it.
Thankfully, the danger of fights is very small. It is mostly very dominant older sows that run an established herd with an iron paw that will reject a boar in their group. Young sows are usually far more welcoming, as they want to form their own tribe.
Don't be surprised if Fred starts losing some weight once he is with the girls - he will be a very busy (but happy) man! Once the girls have accepted him right at the beginning, you need not fear a fall-out ever again.
I wish you the best of luck!
