Put a shout out to agnesf - she had exactly the same situation as you earlier this year.
Her bachelor boy was 'married' up to a female who IIRC was a little bit older but wasn't settling with female pigs.
AFAIK neutered boars take a month to go Jaffa . The neutering doesn't change what's going on upstairs (in their brain) just downstairs. They might still try to mate with a female but she'll most likely tell him to sling his hook.
You need to introduce them on completely neutral territory- so a room where your boy doesn't go, otherwise it's 'his' territory.
When they go to the main cage scrub it completely (steam, vinegar, Hutch Cleaner ) to get his smell off. (Some people use Dettol, but anything like that would need rinsed well after)
You could try swapping a bit of their bedding between the two cages so that they get used to each other's smell.
Your new girl will need quarentined (the rescue will tell you how long for)
And your boy will need time out to let the neutering take effect.
I think fortifiedwithtea has her OB (Old Boy) side-by-sided with 2 females so he gets the benefit of company but he's not neutered.
I kept a boar in one room and a sow+ female piglet under the staircase (huge caged off area, open front) when I was a child. They could chat through the bars but we had to watch them like hawks.
Find out how long your sow will need quarentined then you could let them be in the same (neutral) room with the cages together.
I'm sure your boar will be delighted with the company 
And yay for Rescue Pigs!