How do you help your kids with portions?

My son is learning to cook and is getting pretty good at measuring properly.  So this is helping him learn about portions but the problem is that he wants to go back and back.  Unless  I tell him no that's enough he'll take 2nds of a good meal.  He doesn't necessarily get lots of sweets or chips and we try to eat pretty well at home but I'm worried about him eating out with his dad or school where I'm not there to remind him about not eating too much.  He's trying but as you know when others are eating you want to keep on eating.  His dad seems to have a block where our son's weight control is concerned and is big himself.  So what I need are some ideas to send my son with when he visits.  Has anyone been thru this dilemma?

My son is 10, is 5 feet tall and weights 120 punds. His Dr. said everything is proportinate and fine as long as he is active and watches his portions.

What you are doing is perfect! If he sees what you do he will eventually learn to do it as well. My son could out eat my husband (who is 6 feet tall and 200 lbs) and has once or twice ;-). We just constantly remind him to stop eating when he is either full or we feel he has had enough. I also only give him as much as I eat (since we are about the same size) and tell him to have some water or milk with dinner, this helps fill you up. Then tell him him can have a snack in an hour or so if he really is still hungry and make him have fruit, veggies or something along those lines. 

AS far as school is concerned, he has cold lunch 3 days a week (an we choose healthy items for that) and hot lunch the other days to avoid the over eating bad stuff at school. When we go out to eat, he is already doing really well with stopping when he is full. That used to mean eat it all...now he leaves food on the plate! You can not control what he does when he is with his dad, or at a friends etc..all you can do is reinforce it when he is with you, let him see you doing the same thing and he will be fine.