I see that you have already received some excellent advice. I'll just add a couple of things. It seems like every kid is getting diagnosed with ADHD these days. What I've personally noticed is that I have had to cut almost everything out of my life except mothering so that I can be calm. When I am easy-going instead of hectic, my daughter is more easy-going, too. She gets more attention and nurturing because I have the physical and emotional energy to give her what she needs. As I've watched other women around me, I've noticed that I'm not the only one who tends to take on too much. It's been really hard for me to simplify my life because I feel like I'm capable of so much. But mothering requires a certain amount of mental serenity so that we can be totally in tune with what our children need before they start acting out in order to get us to notice their needs.
Also, sometimes we just need a little education. A book that has helped me understand different stages of child development is "Ages and Stages". I think that if we understand how to nurture and discipline for the various stages of development, our children do much better. Otherwise, we beat our heads against the wall wondering what more we can...because we're doing all we can do...just the wrong things.
Personally, I would not want to medicate until I felt that I had optimized other aspects of my parenting. This requires personal sacrifices and I sometimes think that some parents medicate because it's easier that sacrificing. Nonetheless, I'm not opposed to medicating in cases where the child is suffering from a physical abnormality. Just make sure you're not masking a problem that belongs to someone else. You'll have to take a hard look at yourself. I've had to do that. Frankly, I do it all the time. When my daughter acts up, I try to first ask myself if I've been giving her what she needs. If not, I take responsibility for that first. She's not going to learn and mature normally without lots of guidance from me.
One other quick thought: I wonder if the fact that he DOES listen to papa and nana means that he is capable of paying attention and following instructions. This may indicate that ADHD is not the problem? I don't know. I'm just wondering out loud.
Good luck!