If I was 5'3" and weighed 200 lbs I'd definitely start on a diet and lose some weight. Honey, you are setting yourself up for some dandy health problems if you don't already have them. Diabetes for one, heart disease for two, if you get pregnant you will end up weighing much more after you have a baby and can't get it off no matter how hard you try.
Yes, give me the old I'm happy with what I look like jazz, but I'm not buying into it honey. He wants to be fit. Being that tall and weighing 350 is going to kill him in short order. He has the right idea to want to lose weight. You should also take a good look at yourself in the mirror and seriously re-think your body and what damage you are doing to it down the road.... Not just right now..... But when you are say 50 years old and can't catch your breath as you weigh too much and have heart problems and take a handfull of pills to stay alive every day.
Start on a sensible diet for one thing. Cut out all the sodas rather they are diet or regular. Cut out all the junk food and fast food. They are real killers.
Just because he goes to a gym does not impress me. He needs to combine a sensible diet and exercise or he's just fooling himself also.
If you really want to help him and help yourself, start losing weight now before you get much older. The older you get the harder it is to get rid of the weight as our metabolisms slow down.
I am not being mean, I am just telling you like it is. And yes, he will slim down and get himself a "matching mate" to boot. Gyms are a great place to meet women, if you haven't already figured this out for yourself and the lame excuse of "we can't afford me to go" doesn't wash, sorry.