last year our 11 yr old ate soem of the kids meals. He found the cs meals were quite big but the ts meals were a lot smaller, for example he had a childs pizza in rose and crown and it was really quite small and came with a few bits of apple and carrots ( i think). After that we learned and ordered him some fries and another side to fill him up.
It didn't seem a problem with disney to do this, it just cost us more as we had the ddp but then had to pay extra for all the sides.
My sister (24) regularly orders off the children's menus at both CS and TS locations. They have no problem doing that and they always increase the potion size to adult. We've never had an issue doing that anywhere. You may have to ask for an adult potion though, because he if is young and ordering from the children's menu, they may just take that at face value - child ordering from child's menu = child, (but still charge you for an adult meal!)
In fact, even SW were very good - we all had hot dogs, which were only on the kids menu and should have been served in Shamu lunchboxes, but they just put them on a plate for us, with a nice helping of fries!
There are a few threads on the main board about this and it seems it has never been a problem. SOME CS restuarants (you have to ask at each and it depends on where and largely which CM you speak to at the time) will allow a you to take two small sides as an adult meal. It isn't a hard and fast rule so if they allow it, it is at their discretion and an extra service. I wouldn't try it if the place is heaving though.
Remember, if he orders off the adult menu and has adult DDP, which he will have, he gets an adult dessert as well. So a child's meal and a full sized dessert may well be sufficient for his needs. There are several threads on the main board about portion sizes under various categories and if you have time, it may be worth checking those out. Also, check out the food porn pages with pictures of the food. That should give you a fair idea and you could let your son have a look at it too.
Good to know he can choose off the child's menu but ask for adult portion, and also that they are a little bit flexible on sauces, change veg etc. This will come in handy for some of the adults as well.
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There are a few threads on the main board about this and it seems it has never been a problem. SOME CS restuarants (you have to ask at each and it depends on where and largely which CM you speak to at the time) will allow a you to take two small sides as an adult meal. It isn't a hard and fast rule so if they allow it, it is at their discretion and an extra service. I wouldn't try it if the place is heaving though. 
