It’s so lovely that you aren’t taking the aggressive label as a given and are willing to look into options for him.
Fear can result in ‘aggressive’ behaviour, but incorrect bonding process, hormones/age, plain incompatibility are all factors as to why he hasn’t been successful in finding a friend so far.
The best way to find a new friend is via dating at a rescue centre and being tried with available single males until he finds the right one. However I’m aware that in the US that may not be always be an option. If you do have rescues near you who offer, then do look into it.
Do you happen to know how long he has been alone and his many other piggies he was tried with? Obviously the five months he was at the shop, but prior to that? (Presumably they were only trying him with other single piggies ie they weren’t trying to bond him as a trio for example because that just doesn’t work).
Some piggies though genuinely can’t cope to share their territory with another piggy but even those piggies benefit from interaction with another piggy as a neighbour.
Cross gender bondings tend to be more stable so you could look into neutering him, and after the necessary six week wait for him to become infertile, try him with a sow or two (cage size permitting).
If he really will not accept another piggy, then the best thing is to live next door to another piggy/piggies. Having another single piggy (ideally one who may also be not willing to accept live in company) as a neighbour is a win win situation. Both piggies get the companionship and interaction they need albeit through the cage bars.
Should you only be able to get another bonded pair, then that is also fine for Orion. The pair can live next door and he can interact through the bars (only) with them. A bonded pair of boars can live next door to a single boar without any issue.
He could also live next door to a sow pair if you cannot neuter him, but you must make sure his cage is very secure (ideally lidded) as boars are very determined in the presence of sows and capable of escaping to get in with them. (He is likely to be very excitable if he is suddenly in the presence of sows)
Single Guinea Pigs - Challenges and Responsibilities
Moody Guinea Pigs: Depression, Bullying, Aggression, Stress, Fear and Antisocial Behaviour
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
A Comprehensive Guide to Guinea Pig Boars
A Closer Look At Pairs (Boars - Sows - Mixed)
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?