Hi and welcome!
Would you please add your country, state or (for the UK) your county to your details? the options you have depend very much on where you live. Click on your username on the top bar, then go to personal details and scroll down to location. With members from all over the world, that will help immensely in always giving you the appropriate advice and recommendations straight away.
Guinea pigs should not live on their own. They are herd animals and not wired to live on their own; your boy is feeling very lonely and exposed at the moment. Most shop piggies also have not had much interaction with humans, so they react instinctively by letting a predator "play" with them and worry them without getting much of a reaction in the hopes that the predator loses interest soon and they survive. That is the "nice with humans" behaviour you are witnessing in the shop.
Unfortunately, at five to six months old, your boy is currently at the most difficult age for a successful bonding; you have seen his dominance behaviour in the shop as he is in the midst of his teenage macho hormones (they last from 4-14 months, but around 6 months is one of the major spikes). Yu will have to take that into consideration when you set out about finding him a suitable friend.
You can ideally look for a younger, preeferably non-dominant subhormonal baby boar for company; that will mean that they are never quite at the same stage during the pre-adult stroppy months and dominance is more lear cut, which means that you have a somewhat improved chance of avoiding a fall-out; whether they take, very depends on just how boisterous and hormonal your boy is right now!
Unless you have got access to a good rescue (your location needed for info on that!), you would have to plan for your two boys to live next to each other with interaction and stimulation through the bars if things don't work out. The key to any successful boar bond is character compatibility, but that is the one factor you cannot control when buying from a shop or a breeder - hence why so many boar pairs have an unnecessarily bad track record.
Alternatively, you can consider having your boy neutered by a good vet with experience and a good track record in small animal operating, so he could live with a sow after a 6 weeks post op wait to make sure that he is 100% safe to live with a sow or two (I have the surprise baby courtesy of a supposedly safe over 5 weeks post neutering op boar living with me, just to prove that point).
The latter option has the advantage that cross gender bonds are the most stable of them all, but they come with the especially for America and Australia quite high expense and the not inconsiderable risk of complications depending on the quality of the vet and his operation team, provided that you have access to a piggy savvy vet in the first place. Unfortunately, unlike rabbits, neutering doesn't change boar behaviour or personality, so it won't do anything towards the success of a boar-boar bond.
Please don't rush into things, but do your research first. Company is desirable, but it is much trickier to achieve than you would expect from such a sociable species, so you want to be prepared, especially when your risk-minimising options are more limited.
You may find these threads here helpful:
How to best settle in your new boy:
https://www.theguineapigforum.co.uk/threads/how-do-i-settle-shy-new-guinea-pigs.36239/
https://www.theguineapigforum.co.uk...stincts-and-speak-piggy-body-language.117031/
Tutorial video for pick-up training:
https://www.theguineapigforum.co.uk/videos/
https://www.theguineapigforum.co.uk...or-a-balanced-general-guinea-pig-diet.116460/
Companionship and bonding options:
https://www.theguineapigforum.co.uk/threads/faq-companionship.37654/
https://www.theguineapigforum.co.uk/threads/boars-sows-or-mixed-pairs-babies-or-adults.108944/
https://www.theguineapigforum.co.uk/threads/importance-of-quarantine.108034/
Boar bonding and companionship:
https://www.theguineapigforum.co.uk/threads/boars-a-guide-to-successful-companionship.76162/
https://www.theguineapigforum.co.uk/threads/faq-introducing-and-re-introducing-guinea-pigs.38562/
https://www.theguineapigforum.co.uk/threads/dominance-behaviours-in-guinea-pigs.28949/
https://www.theguineapigforum.co.uk/threads/boar-dating-service-wales.59233/
It is a lot for you and for your daughter to take in right now. Please don't make any knee-jerk reactions that you may rue later on. Even if you end up with two boars living next to each other, you will enjoy the interaction and the difference in personalities more than just one boar.