If you restrict sweet treats then when they get the chance they will gorge. You aren’t allowing them any chance to learn to moderate their intake by being so restrictive. If you treat sweets/chocolate as you do any other food and serve as if it’s no big deal then they won’t feel the need to gorge when they get the chance.
I think sending him to his room is unbelievable, to be honest. Punishing him for eating a gift he was given is horrible behaviour, IMO. You are shaming him for eating chocolate. Shaming an 8yo for anything is despicable, but for eating food is just the right way to set up poor eating habits for life.
Maybe not the advice you are looking for, but in my opinion you are raising them to think sweet foods are extra special and therefore something to be coveted. Follow any child nutritionist on Instagram and they will tell you that’s the absolute worst way to do it. Giving sweet foods occasionally, as part of a main meal, not making them special, is how you teach them to eat them in a moderated way.
It’s obvious you care about their diets, but your attitude to this is very damaging to their relationship with food. Please check out family_snack_nutritionist on Instagram and maybe take her sweet treat course to help reset your own frame of mind on this.
ETA: my kids get very few sweet treats, but when they do they are served with dinner/lunch (eg today they had a chocolate coin each alongside lunch) and they therefore don’t think it’s anything much special. My 2yo didn’t eat all of her coin, and last week my 5yo didn’t eat any of the chocolate he was given with dinner, he ate all his vegetables and said he was full. (And by no means are they perfect eaters, especially mr 5yo)