I suspect you're referring to the main pivot (aka bottom bracket pivot) where on this bike the lower shock eye connects with the seat tube and chain stays of the bike. Most full suspension bikes I'm familiar with have a single bolt thru main pivot. The "bolt" is sometimes called the pivot axle. There are associated sealed cartridge bearings, sleeves, bushings, shims, washers. Sometimes just a plastic sleeve bearing or IGUS bushing is used in lieu of ball bearings contained in a cartridge bearing. This set-up is found on lower quality full suspension bikes. My 2016 S-works Stumpjumper has a total of 10 pieces that make up the main pivot assembly. The center piece (literally) is the axle which is essentially a hollow M15 bolt, 77.5mm long, that spans the width of the bike and threads into the frame's native threads on the drive side chainstay.
It appears you have a type of 4 bar suspension, perhaps a "DW-Link" suspension that will have good anti-squat characteristics. Anti-squat is the resistance to activation of the suspension during acceleration. This would be especially useful here as it appears the bike is equipped with some sort of powered assistance which will put more torque to the rear wheel (and thus more tendency to squat under pedal power).
Powered assist is welcome here as well because it's gonna require some wattage to get those 4" to 4.8" wide tires rolling, especially through or on top of deep snow. Being full suspension brings the bike's weight up quite a bit over a hardtail, so that is a factor in the required power equation. A fit, muscular, 120 kg man should be able to generate decent power, but riding a fat bike through snow--deep or otherwise--will require substantial and sustained power output.
Regarding the increase in tire width to 4.8" I question the benefit of the additional tire footprint versus the increased weight and rolling resistance inherent to the extra width of the tires. Before purchase of the wider tires, I would make sure there are not going to be any clearance issues. Not only laterally (will the 4.8s rub the chainstay?), but also the wider tires will typically add to the overall wheel diameter which will require more power to get rolling and, if there remains any clearance from tire to seat-stay bridge (if one is present, can't tell from the side view photo) that narrowed space will become a choke point for snow and ice to build and rub the tire. Same issues for the clearance between tire and the lower aspect of the seat tube. Also you must think about rear suspension activation and the necessary clearances required. I don't feel the additional tire width would be at all beneficial to this set-up.
Finally, the size of the chainring is ala a road bike. Larger chainring means higher gearing requiring more power. Whatever power your brother can generate supplemented by the wattage of the assist system, is the additional speed possible from the higher gearing necessary or even useful if the bike is to be used to travel on two wheels in winter weather? I'm no pansy, but that situation is worrisome to me even when I can get past the non-usefulness of a high geared fat tire bike, especially off road or thru snow cover.