As others have said really don't say anything about his appearance. If he's suffering from mild depression already that won't help.
Coming at it from the health point of view would be much better.
What CogitoErgoSometimes says above is really good advice:-
''Discourage ambitious plans to shed 50lbs overnight with pills, run marathons etc but encourage small changes that become part of his daily routine.''
If he doesn't have much willpower then, unfortunately, it'll probably be down to you to keep pushing him.
There are some practical things you can do relatively easily. For example, you say he likes curries and you don't eat together all that often. Perhaps find some recipes for low calorie curries and other meals that he likes and prepare a whole batch of them and then freeze them.
This way, all he has to do is zap them in the microwave when he gets home - quicker than going out to the takeaway. Also a lot cheaper than relying on the takeaway and you can make sure that it's low calorie.
Also, maybe have a look at the 5:2 fasting thing:-
www.mumsnet.com/Talk/fasting_diet
My DH and a couple of other people I know do this - well, they actually do 4:3 - and they say that they can cope with this better than being on a diet all the time as they only need to ''diet'' for one day at a time. It's also really important to drink plenty of liquids when fasting, my DH has a huge mug and gets through about 3 pints of tea during a fasting day.
Everybody is different, however, so this may or may not work for your DH.
The other thing is exercise.
Your doctor is SO right about exercise helping with depression. There have been a number of studies showing that regular exercise can be as beneficial as taking anti-depressant pills.
Doing some weight training is just as important in losing weight as doing cardio - in fact some people say it is more important.
If it's a problem with getting him interested then the best thing to get people motivated is to have a personal trainer. Now, of course, these are really expensive. But, if you can get a babysitter for DS then why don't you act as his personal trainer. Not so much in terms of setting up a programme or anything but more as chief cheerleader if you like.
So, you would be there ion the gym encouraging him to do that last rep that he thinks is just too much for him.
There are quite a few budget gyms opening now so if you google ''budget gym'' or ''low cost gym'' you should get some links. My DH and I pay £15 each per month for our gym and it's got everything.
I hope this helps a bit.