I'm 
at your sentence another for company unfortunately we decided to get a rescue pig and he is male
Are you regretting getting a male companion?
I have a similar situation to you. My two rescue boars were nearly 3 (brothers) got along fine 99% of the time. Odd spat but that's to be expected.
About 3 weeks ago the more placid, less 'boary one' died. The other pig was there (as were DD and I) . We put the dead one in the Pighouse overnight with the dominant boar so there was NO mistake- he was not coming back.
Then I had to find a companion. My DD didn't want to have her boy neutered so we looked for a piglet.
I'd done some research (as you do when you have to face the situation at some time)
You need to introduce in a completely neutral territory. I put mine in a brand new paddling pool so no GP smells.
That night we put them both into the indoor cage (which is plastic so I'd steamed it and used cage cleaner) a newspaper base and a folded towel.
My older boar was very protective but we had the Rumblestrutting and bum dragging as you describe. The piglet was following him like a shadow or diving into the corner to hide (Like a speeding bullet). I didn't give them any boxes indoors, I didn't want piglet trapped. The piglet kept going under the boars tummy at first.
We put them outside the next day then gradually introduced them to grass over the next week.
I looked on the Barmy 4 Boars website (it's not been updated for a while but there's a good section on boar:piglet introduction)
They said keep a close eye for the first couple of hours but if that goes well, then they should be fine.
There was one site I read (can't remember if it's the B4B) that you do need a very young boar. Mine was approx 6 weeks (between 6-8) so he had left Mum. There were some that were still with their Mum and though a boar can wean at 3-4 weeks, I don't know if I'd be happy putting it straight in with a large adult.
I know I should have quarentined our new pig but my adult boar was desperately lonely (from the death of his brother to meeting his new boy was a week)
He got very agitated if his pig was away from him. So I thought "He's seen a vet. What's the worst that can happen?"
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Our little boy has been trying to mount the older one a couple of times. Old boar walks away but piglet might get a metaphorical smacked bum. I can visual a tusse in the future- you might have the same. Your older boar is going in his prime and your piglet will be in a couple of months.
I reckon our baby will take over as Alpha Pig, we'll see how it goes.
I'd give your boys at least 24 hours to observe and take it from there. Make sure you completely neutralise everything, clean out the smell from your currant boar.
If push comes to shove I can side-by-side by boars but Touch Wood they are happy.
Did your Rescue give you a "return policy" if they don't get on?