I have two Singapura boys who absolutely adore each other and at the age of nearly five, still wrestle, play chase, then curl up in the same basket or bed and groom each other or sleep together.
They are litter brothers, so I never had to introduce them to each other. They are both neutered, are indoor cats, and have never sprayed.
As far as I'm aware your 5 month old is still pretty young and therefore adaptable; once both boys are neutered there's no sexual rivalry. I would imagine that after that it's whether they are compatable in nature, two 'leader of the pack types' might clash for instance. it might make the difference between two cats who tolerate each other but are effectively two separate feline 'groups' within the same household, or two cats who are 'best buddies'.
If you're using the same breeder, who you say you trust , perhaps s/he can pick a kitten suited to your current boy?