My daughter was in WDW the week before she turned 3 months and then again 2 weeks later for 2 weeks, so she was about 4 months when we left.
You can bring your stroller in restaurants when the baby is that young. We brought ours in to Flying Fish and she slept in it the whole meal. That was the first time I ate a meal without holding her since she was born. We held her at most other restaurants (including the CG) but that was our choice because she was a not a big fan of anywhere other than Mommy's arms at that age. Some places have infant highchairs, they look like those infant seats you see on the carts are grocery stores and they're on a base with wheels. We tried them, but again, my daughter did not care for it, which we guessed since she HATED her carseat at that age and this was basically like her infant carseat. I know Tony's Town Square has them and on our most recent trip I saw them somewhere else, I think CG and Donald's Breakfast in AK.
Since my daughter did not like the stroller much, she spent all 3 weeks in WDW in the Baby Bjorn carrier (she did go in the stroller a bit in Epcot, but in the MK it was more annoying to put her in and take her out at every ride so she just stayed in the Bjorn). To keep the sun off of her I bought a very thin, light colored receiving blanket at Target and stuck it in the top of the Bjorn and then draped it over the back. (Hard to describe, I'll look for a photo when I get to my own computer.) But think of the back of the Bjorn and then instead of seeing the Bjorn you'd see a blanket covering all of it instead. This covered my daughter's legs and arms from the sun and also kept her much cooler (we were there in May and June). She actually would sweat much more in the stroller than when she was in the Bjorn. I also put a sunhat on her, also from Target, which was very lightweight and had a brim in the from and a flap to cover her neck in the back. I wore her facing me almost always because I could keep the sun off of her better. I also used this same idea on our trip in December when she was 9 months old, but to keep her warm. She faced out then, but I tucked a blanket in the front and then wrapped it around her so she stayed warm and was protected from the wind. Not sure when you're going so I thought I'd offer both. She was 12-13 pounds at 3 and 4 months and I had no problem wearing her all day in the Bjorn. At 18 pounds and 9 months it was a bit harder, but she still was in the Bjorn more than her stroller. At 6 months in DL, I ended up holding her in lines more than using the Bjorn, only because she was in her stroller a lot and it was easier than taking the Bjorn on and off and the lines were short.
At 3 months we took our huge Graco Coachrider stroller because it fully reclines, the 'seat' is removable so we could also use it like a bassinet (great for when she fell asleep in it and we could just keep her in it and take the whole seat out), plus it has a big canopy and it's reversible so she could face us (no way she'd stay in it at all without seeing Mommy). It was great for her at that age (and we also brought it to
Disneyland at 6 months) but was huge, heavy, and so inconvienient for the buses. We also took it to Disneyland when she was 6 months because we didn't need to use buses ever and she was quite happy in it and stayed in it almost all day, while it was facing me. At 9 months we brought our Maclaren Techno Classic, which also reclines fully and has a canopy, but only faces outward. She liked it (and I did too as it's so light and I could carry the stroller, diaper bag, and daughter in the Bjorn onto the buses myself) but it was harder to keep the sun off of her. We have extendable canopy attachments but forgot to bring them. We also bought the Organizer for the back, which is like a lightweight add-on that has cup holders, plus a pouch for a cell phone (not that you'd leave it there on rides), and a large pocket which we kept diapers, wipes, toys, blanket, etc. in. Made it much easier than the stroller by itself. I'd use whatever stroller the baby likes, otherwise he might not want to go in it much.
One other suggestion is to use the baby care centers. We used them a lot as they're a great place to feed/change the baby in a quiet, cool environment. They have private rooms for breastfeeding and real changing tables. I used them to give my daughter some tummy time at 3-4 months and as a place to crawl around at 6 and 9 months. They're great to just take a break from the hectic pace of WDW.
Have a great trip with your nephew-it's a great age. Oh, my daughter loved light shows at that age (still does), so definitely see Spectromagic if you can. Be sure to buy lots of cute little Disney outfits while you're there!
