I thought I would chime in here. Our kids our exactly 2 years and a day apart. My son was born on Dec. 28, 2 years ago. He is also a chunker like yours. He was 16 Ibs at his 3 mo checkup. He's now a little over 2 and he's 35 Ibs and this is only the 75th percentile. Let me tell you, there would be no climbing Mt. Rainier this year, although we could have gotten away with it when he was 3 mo old.
We used our Ergo on me in front the first year with my pack filled with light stuff on back. Last year because of some medical complications, I broke my arm in Spring and hubby needed surgery in July, we only got to do one backpacking trip. We had me with my son in the Deuter, and hubby carried almost all the gear.
This year, I am planning on only 1-3 mile hikes in. Ds simply too big to carry with gear for very long at all, but he can't really walk more than 1.5 miles. We will base camp and bring the ergo with us for hikes from camp. If need be, I figure we will drop one of the packs, put my son in the Ergo, go to camp and one of us will go back for the second pack. With some of my friend's lighter children, I think we could get away with the Deuter again this year. To me, the Ergo is a lot better once they hit the heavy stage because it both holds the weight more centered and because it is much lighter itself.
This is all just for your information, since the Deuter is so expensive. It was worth it for us because I also lead family dayhikes for our local hike club and needed to be able to bring a first aid kit, etc. But I'm not sure it would be worth it for everyone, if their kid was on the bigger end and wouldn't comfortably fit weight-wise for that long. If you are expecting your dh to carry your son and his gear, he will rapidly go over the 40 Ib mark.