Hi, I figured I'd chime in as an actual "giant". I'm sure some of what I say will not be what you want to read, but I can only tell you my experiences. I should point out that the vast majority of children, even the big ones, are unlikely to get to my height, so I wouldn't worry about that specifically, just see it as more broad take on being an abnormally tall person.
I'm 6'9/205 cm and well into the obese category, think Hodor from GoT or Hagrid. The two aren't necessarily connected, I come from an overweight and sedentary family, I have a largely sedentary job, and as an adult I use food as a crutch, so it's safe to assume I would be overweight regardless of my height.
My height never really held me back in school. In lower school I felt no different from anyone else, nor was I treated as.such by my peers.
In middle school things began to change, mostly because of my weight and the fact I was quite "soft". I remember a couple of accidents in lower school (genuine accidents I should add) when playing at break times, where it became obvious I could really hurt my friends without meaning to, such was the size difference. As a result, I shied away from physicality. When the bullying began, I wouldn't stand up for myself for fear of getting in trouble, so I became an easy target.
It got much worse once hormones kicked in, and boys began to take an interest in girls and vice versa. Teens still think very primitively when it comes to trying to attract someone, and several of the lads saw me as a scalp, take on the big guy to impress the girls. It's quite sad how often it seemed to work aswell. It didn't help my self esteem at all, that it was also becoming quite apparent that the girls among my year group seemed to view me as a freak. Certainly not someone anyone wanted to go out with.
I entered my teenage years despising myself and hating the people around me. My home life was also less than supportive, and I believe this to be the key. With a safe home, where I knew I was loved and valued, it's possible the things at school wouldn't have got to me so much. As it was, my parents marriage was slowly dying its death over a number of years, and my Dad reinforced the kids at school by telling me that "no girl will want you if you stay that fat" and things along those lines.
It took me many years, and going to the brink of some very dark places, for me to accept who and what I was and to see myself in a positive light. I dropped out of college, and never went to university. I never had teenage love, or any of those experiences that so many see as standard or take for granted.
The reason I say love and support at home is key, is that there were two of us in my town. Both of us the same height, both of us very overweight, and we both moved in similar circles so we were aware of each other. I was defensive, cynical, depressed. He was outgoing, well liked. Neither of us did very well with women, but then, not many people are attracted to fat people at the best of times, let alone when you add abnormal height into the mix.
I'm not sure how it happened anymore, but around 26/27, I finally just accepted things and let go of all the self loathing. I'm still a cynic and I'm not a fan of people, but I don't hate MYSELF. I met an amazing woman a couple of years after that, and I'm now the incredbly proud father of a wonderful, beautiful little girl who is my world. She's already in the 99th percentile for height, and it genuinely scares me what awaits her as she gets older and goes to school, but she will always know how much I love her.
I still struggle with my weight, even moreso now I'm getting older. I've tried everything bar a gastric band at this point, and I just don't want to take that leap. Don't put your son on a "diet" and never tell him he has to eat a certain way "or else he will get fat". If he is naturally slightly heavy set, he will see himself as "wrong" for not being able to match up to the ideal, if he's taught that fat is negative. Instead, encourage an active lifestyle and healthy food choices. Get those routines and practises in his head young, and they will stay with him into adulthood. The younger you do it, the better it will be for him, and always, always show him love and that to you there is nothing wrong with him, height, weight or otherwise. Make his size a positive aspect of his identity, not something to feel ashamed of or to shy away from.
I am an exceptional case, in that my life path was the product of multiple negative things happening at the worst time. I'm the extreme end of how things can turn out and I know the vast majority of tall people will have nothing like my experiences, so I wouldn't overly worry.
Apologies for the long winded reply. I saw this thread and it brought back alot of shit for me that I felt the need to let out.