Hi Sven
I was in this situation a couple of years ago with our adult boar when his brother died. (There's a thread back in July/August 2013)
We got a very little piglet from a Rescue (6 weeks old).
I didn't quarentine him (my bad) because our adult was lonely and piglet had been through his vet check.
(we later bonded the younger boar with two sows then in May we got a new boar when he died. But we had to seperate while they went sterile)
Neutral bonding territory. Get a cheap paddling pool to bond them in. Towel or newspaper and food.
If you phone some Rescues they'll tell you what they have. There may be an older male that will boar match but a piglet is an easier bet.
They;ll put them in a neutral space to let each other suss the situation.(Our boar was instantly enamoured of GP3)
Then put them for an hour or two in the paddling pool and watch them.
We;ve put ours in the Pighouse but I deep clean with a steamer and VirkonS .
That's THE saddest thing, removing all traces of the piggie that has died 
Change things round a bit. New boxes. Two doors cut in the boxes.
Spaces between the foodbowls.
When we got the sows (and then later when we got our newest boar) the new arrivals stayed in the indoor cage which we put inside the Pighouse so they could chat but not meet.
Then the bonding as before but it was 2:1 
Pigs generally like company.
And we on SmallPets demand photos of new arrivals 