If I have a Marine who hid his diagnoses from the Marines, and his ADHD is now affecting his performance, what can he do for help?

So he lied. On official paperwork.

If he has admitted fraudulent enlistment, as an NCO you are obligated to report it. More importantly he needs to get help before someone gets hurt. If an accident were to occur due to his inability to focus, you will be in a world of shit. Unfortunately the best thing you can do may seem like the worst thing for him.
Again, I completely understand the predicament you are in, but you must report this to your immediate CoC so it can be dealt with. This is not something that can be handled at the Team or Squad level.

Previously ADD or ADHD was automatically disqualifying, but today it is only disqualifying if the applicant has been treated with ADD/ADHD medicine within the previous year or if they display obvious signs of the condition. With that said, if the Marine in question WAS taking meds within the year previous to enlisting....yea....that makes this fraudulent enlistment.

Now, you are seeking information to assist this Marine to keep this fraudulent situation going so as to NOT hurt their career? So much for Marine integrity.

This will all eventually come to light down the road and some ass cheeks are gonna get burned.