Both of my kids were about the same size as your son but built much differently. My poor son has almost no rear end, and my daughter is on her way to a larger one.
We had problems with all diapers when blow outs were happening. It wasn't the shape of the diaper (we tried to go with those that had elastic waists in the back) - but it had to do with the quantity of the poop and the ability of the diaper to wick it away. Unfortunately, some blow outs are just common, and we'd keep extra clothes on us at all times and at Day Care just in case.
I'd stick with what works the majority of the time as no diaper will be able to contain everything. At 2, our daughter had a blow-out last night (all over my arm) at the park. It's easily been 6 months since the last one.
Just to share a chunky baby story - our daughter was born average (7 lbs, 8 oz) but quickly grew to a BIG baby. We were in Chicago visiting an Oncologist, and in the elevator, one of the employees commented on her "juicy thighs". I had to start calling her Munchkin so Chunkapoo wasn't the nickname that stuck.