Hi and welcome
I am very sorry for your loss.
Fact is that is more than half of unmatched same age boars actually make it through teenage together and that percentage is going up even more if one of the boars is an adult because that way there is only one boar hitting the hormone spikes at any time, which goes actually quite a long way towards stabilising a bond although it is not a guarantee.
The fail rate is much, much higher in same age trios and near 100% in quartets, which is the most unstable boar combo of all during teenage.
The fail rate in same age baby boar pairs is rather high because the all important personality match is ignored when piggies are chosen at the shop - people buy for looks and not for which boys are hanging out together. Bringing home a new friend on spec is also unfortunately somewhat of a gamble. Other factors like too small cages, huts with only one exit, shared bowls and only one water bottle and hay access etc. can act as additional flash points when the under-boar is unable to remove themselves from the premises as they would normally do so they are compelled to fight.
Crucial is initial acceptance during bonding as to the presence or absence of a personality match and then later on whether the boars are still compatible and want to be together when your baby boar is developing his adult identity during teenage. Key is generally how dominant the baby is in his own outlook when he comes fully into his own.
A not very dominant baby boar will simply fit in and can often create a very loving bond with an 'uncle boar'. Once weaned, baby boars are wired to associate with a larger bachelor boar willing to take them to learn the ropes. However, if your boy is not OK being a bereaved single, then chances are pretty high that he will accept a baby boy and take him under his wing.
Just make sure that you have a cage you can divide if necessary either temporarily during a teenage spike or for the long term so your two boys can become a 'can't live together but can't live apart' boar pair that retains their bond with full interaction through the bars but each with their own territory as your plan B, which will hopefully never become necessary.
My Nye was ca. 4 months old when I adopted him as a companion for my very skittish adult Nosgan after he turned out to be total fail with any ladies of mine and not just those of his previous owner. I went through the whole teenage with young Nye but the two boys only needed a short temporary separation twice, both on days I was dealing with a proper female pheromone storm from several sows coming into a strong season together in my piggy room. I had Nye neutered at 6 month (he was considered still a bit too small for an op before Christmas and had to wait an extra month) but he still went through all the typical teenage phases. He did go on to live happily with a sow pair of mine after Nosgan's death who they had been living right next to for several months - even though it is generally a big no no. Since Nosgan was not really interested in the ladies, Nye and Hyfryd were able to fall in love through the bars without this causing any problems.
You may also want to read these guides here for some more balanced information and a deeper understanding of how boars work and where they are coming from:
A Comprehensive Guide to Guinea Pig Boars
A Closer Look At Pairs (Boars - Sows - Mixed)
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
The Herd, the Group and I: Investigating Guinea Pig Identity and Society (there is also a chapter on boars which you may find illuminating)
Journey through a Lifetime: The Ages of Guinea Pigs (looks at the development and what happens at each age, including pre-teenage and teenage)
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