Hey Fairy,
I can completely see where you're coming from, and I can tell you're being a fantastic mum by not pushing your son into anything. I remember my mum telling me I needed to watch my weight age 11 and was completely heartbroken by that. I have a half sister who's seven and she is quick to tell me that she is getting a tummy as all she eats is pasta. I, in no uncertain terms told my mum not to give her the same hard time, as I remember how much it destroyed me for my parent to say that kind of thing.
I too was doing slimming world before this plan, and really loved the support of the group. I'm still toying with the idea of going back once I get to maintenance, but am worried I will feel like a fraud for not having the willpower to stick to a more open plan. If you're still in contact with the consultant at the local group, why not ask them if it would be ok for you to go just as a "driver" for your son, so that he still gets to enjoy, but you're not involved in their plan? That way your son gets to learn and enjoy slimming world and you will still be involved in what he's doing. It might actually be pretty beneficial for you to just listen (I know it would help me) and think about using it as maintenance or something?
Just my thoughts on it and I know exactly what you mean - healthy eating for youngsters is a much better thing to focus on.
xxx