I had a short term 26 mo-old FS that weighed more than my five-year-old. EVERYTHING is more expensive with larger babies; you may want to ask for a special rate. GoodNights are okay, but the do leak if left on just a touch too long. Depends for small adults will work well (but expensive) or try Luvs or Huggies size 6. I've noticed they run a tad bigger than other brands and are okay for leaks.
Don't go by age on the car seat go by weight unless height is a problem. Of course you'll want to keep some of his regular foods handy (so he doesn't lose bfamily and good food in same week) but also some good foods that make them full faster but still taste like junk food are: Apple slices with those little cups of caramel in the produce department, string cheese, Gensoy (flavored protein rice cake type snack) taste like chips, banana bread and banana chips, add shredded lettuce to all his favorite hamburgers and sandwiches, offer grapes and berries, mine really took to frozen berries with no sugar added, natural fruit roll ups, yogurt with extra fruit instead of icecream and syrup all that kind of stuff you already know. That way little one doesn't notice his new diet is lower sugar, lower fat, lower salt. Mine EXPECTED Cokes for breakfast and chips every single time he saw a vending machine.
But it tapered off some.
Instead of a highchair I used a booster seat with a tray that pulls up to the table, but the stroller thing was solved with a wagon. The big Fisher-Price plastic kind with seats that fold up or can be laid flat. He was already used to a twin bed with guard rails and here he got a toddler bed with side rails.
I got used to the physical part of him. The hardest part was strangers (park, zoo, grocery store) that saw a 5 year old NOT a 2 year old and expected him to act and talk accordingly. Get used to the looks
when your "5" year old can't speak clearly enough, tantrums, tries to hit other
little 2 year olds when he's frusterated and can't communicate well enough, ect.