I'm a 'honorary Uncle' for a child who is 5 years old. The child has not been diagnosed by a doctor yet, but as someone who is ADD himself and has some experience volunteering with kids, including ADHD kids, I'm about 98% confident the child is ADHD. It's a particularly severe, and obvious, case! He can't even play hide & seek with his friends because he forgets he is suppose to be seeking kids and wanders off seconds after he says he wants to play. The son's sperm donor also has ADHD so the child was already at a higher risk of having it. I've already informed his parents of this.
I first told them a few years ago, and their response was that he is too young to test, but they will be ready if he is ADHD. I've more recently mentioned it to his father in passing during a visit. His father's general response was that it doesn't really matter until the child is in school so he can worry about it then, though I think I also caught a hint of doubt (denial?) that his son was ADHD as well. I know he hasn't bothered to take his son to get an official diagnosis on rather he was ADHD.
My honest opinion is that waiting is a bad idea. It can take some trial and error to find the right ADHD medication for a child and it seems to me that it would be best to do that trial and error now, before he is in school, so they know he has a good medication when school starts rather then trying to find the right meds and dosage while he is struggling in school. In addition his ADHD is severe enough that I can see it interfering a bit with his playing with other children, both because they don't know how to deal with his level of hyperactivity and because he is too distracted to stick to playing one game with the children for long. As such I feel getting his ADHD a bit better managed would help him with developing social skills now. It would also likely make his father's life a little easier, but that's less of a concern to me then the child's well being.
On the one hand I'd really like to encourage the father to put some serious thought into doing something about his son's ADHD, as I think it would be best for the kid to start sorting out medication now. On the other hand I don't want to impose on the father or make him feel I'm judging his parenting or trying to tell him how to raise his child. It's just that I do have a bit of experience with ADHD and feel I have some insights that could be useful for him, if I knew I could talk to him about it.
Ironically despite my visiting my 'nephew' semi often I don't get to talk with his father too often. Between the father being heavily introverted, to the point I don't feel like I should be forcing him to have a long conversation with me, and my nephew's excitement, and hyperactivity, when seeing me leading to him immediately dragging me away to play we don't get much one on one talking time during my visits. This means any real conversation about his sons ADHD would have to be premeditated and would feel out of place enough, relative to my usual visits/babysitting, that the father may feel I was putting him on the spot just by trying to get a moment to talk to him. The mother is no longer in the picture, so I can't speak with her instead.
So should I bring the child's ADHD up, or just let it go and wait until he starts school? If I do mention something how can I do it in a way that doesn't risk upsetting his father?