If he really is unhappy by himself, then he will need a new friend but you won’t be able to wait until he is through his teens as that is another six months away (you said in your other post they are 8-9 months old).
Do bear in mind that he isn’t lonely as they can interact through the bars but it may be he will be happier with a new live in friend.
While can be trickier to bond teenage boars but all bonds come down to character compatibility, so it doesn’t matter how old he is, he still needs to find a character compatible friend. If you can take him dating at a rescue centre then he can choose his own new friend. If you do get a piggy on spec from a pet shop for example, you will need to have a plan b in case the bond fails, and then you will have three single piggies.
An alternative is to neuter Momo, have his six week wait to become infertile and then bond him with a sow. Again, via dating at a rescue is best.
If you get Momo a new boar friend, then their cage needs to be 180x60cm. If Pua is to remain single, then his cage will need to be side by side at all times and measure a minimum of 120x60cm.
If you get Momo neutered and find him a sow then their recommended cage size would be 150x60cm, again with Pua having a 120x60 side by side. However, in this case you would have to be sure Pua’s cage is incredibly secure and has a lid, as he will be highly tempted and determined to escape the cage and get in with the sow and Momo (which would cause a fight).
Another alternative, is to get them both a new friend each - either neutering them both and finding them each a sow; or taking them both dating and finding them both a boar. If this were to happen, then again each boar pair needs a 180x60 cage or a boar/sow pair a recommended 150x60, but as each piggy will have their own live in friend, then the cages can be stacked one on top of the other which will save floor space and as each piggy has their own friend, then nobody will be lonely.