So, I've noticed that Jett seems to be getting a bit left out like last night he wouldn't eat veggies much- I'm not sure if this is because he is still timid and scared or whether its because he was pushed out. Again, when the other 2 went in their hut he didn't go in he just sat looking scared next to it. The grey and white one also keeps mounting the others which I'm assuming us dominance?
I may have found a 9 month old boar who used to live with a young guinea pig who unfortunately was put to sleep at just 5.5 months old. Would you suggest putting Jett with the older guinea in a separate cage but same room? If so, so you think he'll be okay being split from his other brothers and still being able to smell them? If I do this, I'm guessing it would be best to be done sooner than later?
Thanks
If you are new to piggies, I would VERY strongly recommend to rather taking Jett to a good rescue that knows what they are doing bonding-wise. Please don't compound problems by ending up with boar #4 who won't work out with Jett, but Jett will not necessarily be accepted back with his brothers... mallethead
At the moment, your boys are simply going through the hierarchy sort out. Personally, I would wait until your boys hit the hormones and see how that is changing the dynamics then. That will give you a much higher chance of minimising any potential separations.
I would however, keep an eye on Jett. please weigh all your boys once weekly (as you should any healthy piggy) and see how Jett is doing compared to his brothers. Make sure that you have all in threes, hideys, bowls and water bottles, so that no boy can push the others off their food.
As kind as it would be for the bereaved boar, bonding is simply not quite as easy as sticking two boars in a cage together and they will get on. Things can go haywire in seconds. The boar you are thinking about is at the height of the hormonal phase; believe me, that is a completely different kettle of fish you are dealing with there! Not every piggy will go with any other; character compatibility is essential, even more so in boars.

Just to give you an idea of what can be involved, have a look through this blog by a very experienced rescue lady who offers residential boar bonding at her rescue. A lot of successful bonding depends on correctly reading the body language and the situation:
http://www.theguineapigforum.co.uk/showthread.php?59233-Boar-Dating-Service-Wales
Principally, Jett can live in the same room with a new friend, either next to the cage or above, depending on the space available. You may want to but an opaque divider between the cages if the two couples wind each other up too much.
Here is the Piggy Bank list of recommended (and vetted) good standard rescues:
https://maps.google.co.uk/maps/ms?msid=209994852075231951564.0004b8fd9391b4257d8eb&msa=0