Our kids have been at 7 months and 3.5 and then again at 15 months and 4. The next time they go (Dec. 2006) they will be 5 (6 in 6 weeks), 2 (3 in 4 weeks) and 15 months. I didn't find it any more work than a normal day. Easier actually in many ways.
I didn't mind having a stroller; a lot easier than trying to keep an eye on excited, roaming kids. I'm actually (and I know it is a huge minority) going to miss not having a stroller as the kids get older. It's a place to shove your stuff. Cuts down on whining. Strollers were invented for convience, and I think it works well.
If our kids our tired (even DS#1 who doesn't nap), they'd nap. Right were they were at. There was less whining than when we go to the mall or grocery shopping b/c it is a heck of a lot more fun and interesting. They can go on any rides that aren't height restricted--way more than not. Our littler one certainly wasn't into the "I want's", didn't buy him anything this last trip out at all. There are plenty of ways to take down time without even leaving the park (Tom Sawyer's Island, The Bone Yard, RR at MK, RAfiki's Planet Watch, Festival of The Lion King, Playhouse Disney). Bathrooms are a plenty, fairly clean. You have the baby care centers if you need them (we really didn't use them). I think it is more people's attitude's and fears that hold them back than the actual trip.
I think the big thing is going when it isn't too hot--avoid end of May, June, July and August--which you're doing. And going when crowds are low. Again, something you are doing. It is so much fun for them at this age too because it is all so "real" to them. You do need to be flexible and be willing to adjust to a littler ones needs--many of them change their schedules while at Disney, so I wouldn't want to force the at-home schedule unless that is what works best while there. As long as you go in positive, it will work out wonderfully.
However, in answer to your question. I don't think you can convience him that taking a two year old will be just fine. You just have to convience him to take the chance and with the other two getting older, that might be your best arguement. If he thinks that is the great age to take them at, two out wiegh one.
