My son was sent to a psychiatric hospital both in elementary school and middle school. He started doing better in HS, got into ROTC ( 4 yrs) and was accepted.
A lot depends on supply and demand. Right now, Companies are not able to fill their vacant positions, so they are offering higher wages. I would imagine the military is having trouble filling its vacancies. During times of war, some standards are relaxed, so more people can go in.
Much will depend upon how well you are doing. I don't know if they will take you if you are on meds. Most of the cases I am familiar with are people who managed to function without meds, although they were on them when they were younger.
But seriously, don't lie. It catches up to you. If you are thinking about going in, you might talk to your parents/doctor about trying to go off the meds and see how you do. But, don't stop them on your own. You will need medical records to prove when you discontinued them. You might also talk to a recruiter to see what advice they have. You won't be the first person he's had that has ADHD.
Thank you for your interest in military service - I hope it works out for you!