All of the 'politically correct' labels being debated are actually very different things. Don't use a term that's inaccurate because you think it's kinder or more sensitive. It just makes you look and sound awkward -- like you're trying to be nice instead of just being nice. You can easily be respectful of people with disabilities while not sugar-coating the disabilities.
- Learning difference can mean visual-spatial, auditory or kinesthetic learners and is so imprecise and fuzzy as to be practically meaningless. I know some positively brilliant people who learn differently -- but that's not what you mean, right?
- Learning disability can be dyslexia, dyscalculia, ADD/ADHD, auditory processing disorder, short-term memory deficits -- the kind of thing you can teach around or potentially overcome, and again, without saying anything about the person's base intelligence.
- If you mean there is an intellectual deficit or disability, just say so. It is what it is and calling it something else just distorts your message.
- And if it's a developmental disorder - autism spectrum disorder, Down syndrome, Fragile X -- call it a developmental disorder, because again, that's what it is.
- One more thing - If the person is never ever going to 'catch up', then it isn't a "delay". Don't call it one.
Just my two cents: it's much kinder to avoid applying (whichever term you decide on) as an adjective. It seems more like a label being inflicted when one says "IDD patient." Instead one could acknowledge the disability as incidental to the person by saying: "patient with IDD." In other words, avoid "intellectually disabled toddler" and choose "toddler with an intellectual disability." Instead of "developmentally delayed child" perhaps "child with a developmental delay." This can be more broadly applied to chronic medical conditions as well.
Using a noun rather than an adjective+noun -- ex. "an autistic" versus a "young man with an ASD" is insensitive, crude and objectifying, but placing the adjective before the noun -- "an autistic man" is not really a problem. Sure, all other things being equal, a "young man with autism" is slightly better than a "young autistic man" -- but sentence construction can quickly get convoluted if you put too fine a point on this one.