When my GP3 (huge Rex boar) lost his stepdad , he was just under a year old. We didn't want to put a tiny piglet boar with him (he was lovely but very full on and about 3lb weight)
I got him 2 sows ( about 10 months) who had just weaned their piglets , so they were a tad wild (they'd been part of a Rescue of over 240 piggies)
We had him castrated and put them side-by-side while he went sterile and they got to hear each other chattering.
They were 6 weeks side-by-siding, we put our 4x2 cage in the Pighouse and made sure he was taken out first and put in last so they were safe in the cage , but in case one managed to escape , he wasn't waiting 
We did the same when we got GP6 (GP3 died and we all missed a boar) he'd been neutered two weeks prior so he only needed a month side-by-siding.
But - our next two sows wouldn't bond with our pigs and it was only when we were down to one boar/one sow that they got together. GP8 wouldn't bond !
(The last pigs didn't ever breed whereas our first sows had before we got them. Don't know if that made any difference )
Is your boar already castrated ?
You need the complete neutral bonding area, huge pile of food.
A bath for them all to make them smell the same . Or you can rub some dirty bedding on from the other pigs to blend their smells.
Sows are feisty blighters , much messier than boars IME . Ours were quite happy to kick out or pee backwards if the boars got too "WayHay"
Mostly it was hopeful purring and rumbling (though it could just be that our neutered boars were very Under The Thumb )