Here's the thing- when young snakes are eating young prey items, those young rodents are more digestible. When a snake doesn't need to eliminate wastes, they don't, because it wastes the water in their body too- they save it up until they actually NEED to go, like when you're holding them while wearing your "date clothes"- you know, the "dry-clean stuff"-

(Okay, I'm kidding, it's just a coincidence...I think?)
Seriously, snakes do not necessarily defecate once per meal eaten- it can easily be 1:3 or 1:4- there's no rule, but IF your snake is looking bloated or has a hard lump (urate crystal stone blocking the cloaca) then a soak or just some handling may help. BPs tend to be fairly inactive, & that IS why they may defecate on us when we least expect it (but should have...)- it's the motions* that help, & the same is true of a shallow tepid water soak- it's more effective if the snake can spread out with your support & full attention, & be swimming in place. It's mostly the motion that's helping, not so much the water. *That's also why many snakes defecate while shedding- sometimes right inside the shed (so "tidy"!)- it's because the act of shedding is exercise for them, using all their muscles to "un-dress".
Can you post a few pics showing this "bit bloated in the rear of his body"? Keep in mind that IF you try to soak him prior to a meal, it may put him OFF eating, due to it being stressful.

So unless you really think there's an issue, I wouldn't soak him prior to feeding. In fact, if you really think there's an issue, then you shouldn't actually feed him either- snakes did not evolve with feeding schedules where prey animals voluntarily show up to be eaten on such a regular basis as most of our pets enjoy.