I don't know what's going on as we never had this problem.
From the sounds of it, you're doing everything right with the diapers. I know boys tend to soak the fronts, and girls the backs. Esp if you have a tummy sleeper I think that's adding to your problem.
Perhaps you can cut down on his liquids a little bit and push more table food? I realize he's only 8mos old, so I wouldn't expect him to eat a lot, but if he's soaking through the diapers...
My kids were never 25 lbs at 8mos old, they were always tiny little things. So I'd guess that you've got a good little eater. I wouldn't think that cutting back on his milk would hurt him at all as long as you are adding in some solids like bananas, cereal, pureed veggies, etc.
Since our kids were always tiny (in weight) but nursed too, we added in the solids trying to help fill them up so they'd sleep more. Dr said that some kids just metabolize breastmilk so fast that they eat and eat and eat. The solids did definitely help them to sleep longer and get more calories from food instead of liquid...which means fewer diaper changes, esp at night! That's why we started with "dinner" as the big meal. It also started training them to learn to sit in the high chair during dinner, watching us eat, so I could get a decent meal too. (Dr said watching us helped the baby to learn "It's dinner time" even if they just sit there for 10 minutes smearing food on the tray.)
I am a huge nursing proponent. So I am not trying to dissuade you from nursing. But I really think that 8 mos is a good time to start feeding dinner, so that you and him get the solids you need.
I hope this idea helps!