Yes, Dyslexia is a language-based learning disability.
No, not in a tax context.
a) The law defines disability as the inability to do any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last for a continuous period of not less than 12 months. To meet this definition, you must have a severe impairment(s) that makes you unable to do your past relevant work ( see § 404.1560(b)) or any other substantial gainful work that exists in the national economy
Why---what are you trying to do?
Do I get a credit or deduction if my child is disabled?
If your child is older than 12 and you need to pay someone to take care of him while you work, you can say he is disabled and you can still get the childcare credit on your federal return. Or if he is 19 or older you may still be able to claim him as a qualifying child dependent on your federal return.
Aside from those things, there is no federal tax advantage to saying that your child is disabled. Your state tax laws might be different.
For disabled adults:
There is an extra amount added to your standard deduction if you are legally blind. Your federal return does not provide any other credits or deductions for saying you are disabled, with one possible exception.
There is a credit for the Elderly and Disabled which is so small that very few people benefit from it—-it has not been updated/increased by Congress for decades.
Probably not
Here is the IRS definition of being disabled
Permanently and totally disabled.
You are permanently and totally disabled if both of the following apply.
• You can’t engage in any substantial gainful activity because of a physical or mental condition.
• A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death