Hi!
I am ever so sorry for your loss!
Five is the very bottom end of what can be constituted as a boar group and not necessarily to be recommended to attempt in a big merger with teenagers or several boars in their prime in the mix.
Boar groups generally work better with all adults and much better with older boars (where the hormones have run out and they behave more like sows as the comfort of company of other piggies becomes more important than social standing - but again, it won't work for every boar) or in boar groups with a very different disability/carer companion dynamic. It is however even then still very much dependent on the actual character mix and how many dominant/submissive boys are being socialised; that is why you can never make any prediction on the individual situation; our recommendations rely on trends.
It may well not work out because it all comes ultimately down to as to whether the boars want to live together or not and can decide on just one leader (not every boar, especially in their prime around 2-4 years, is willing to give up their status - it all depends on the mix of how dominant they are in outlook and how personality compatible, too).
Alternatively and with a lot less risk, you could move Percy in an adjoining pen with grid separators that allow full social interaction and stimulation through the bars but still give him his territory. Quite a few members have this kind of 'can't live together but can't live apart' system with a separator where the 'neighboars' are actually still sharing a bond. I would recommend that you see first how Percy is doing next to your older pair and then take it from there in due time some weeks or months down the line when you can try them in a neutral herd feeding setting on the lawn - never rush these things. If he can make friends through the bars, then an adult trio that has chosen themselves can and does work out.
A single full boar can also live happily next to sows as he hasn't got anybody to fall out with, by the way if you also have sows. You have to make sure that he cannot get at them (cable tie the grids and nothing that helps him to climb/jump over the grids, but generally it is not the worst long term alternative and gives him the opportunity to express a lot of entirely different social behaviours and interactions that he doesn't have as a 'bachelor'.
What doesn't work and why we have put in the strong warning about boar groups are baby boar trios and quartets that have been chosen for their looks and not who they hang out with and that all hit teenage at the same time - a perfect recipe for disaster unless you are very lucky and have ended up with a perfect personality balance; but that is rather a minority and not a majority, unfortunately.
If you find a firmly bonded boar trio or quartet in a good welfare standard rescue, then you are always on much safer ground because character compatibility and stability of the bond has been taken into consideration.
More information on next door living alternatives:
Single Guinea Pigs - Challenges and Responsibilities
A Comprehensive Guide to Guinea Pig Boars (read both chapters on age and companionship)
PS: Unfortunately, you have to wait until May until my current article series 'The Herd, the Group and I' for Guinea Pig Magazine about social identity and the complex multi-levelled society and social interaction that they have as a species ends with a look at boars from a species perspective and gets a bit deeper into the 'why' of their specific dos and don'ts rules, including a bit more of a differientated look at our pet boar social options (planned issue 68).
I have so far dealt with how guinea pigs identify themselves, self- and social perception (issue 63); herd behaviours (64); and group aspects (65+66). My new instalment out in March is about territorial group interaction and territorial behaviours that we see (but all too often misinterpret) in our pet piggies (67).