Well, that is a place to start. Meat has essentially zero carbs and also is very filling. I know it can also be expensive. But watch for sales and stock up when it's cheaper and put the extra in the freezer.
It's also great that he'll eat green beans. Green beans are very low in carbs and quite nutritious. If that's the only green vegetable he'll eat for now, you should serve it many times per week, practically every dinner. You can also experiment with introducing other greens once or twice a week. Make a great salad, add to it some diced some ham and cheese in it, perhaps a hard-boiled egg (if he likes those), or other things he might like. Experiment with different salad dressings to help him find what he likes. Dark leafy green vegetables are so nutritious and also do a great job of helping you feel full so you don't overeat.
Also, you can try some green veggies that are kind of similar to green beans. Asparagus, edamame (soy beans—look for them in the frozen foods), and peas are a place to start. Plus, look for recipes that combine vegetables with meat. I know as a kid I never liked broccoli until I had a Chinese beef and broccoli dish. To this day I only eat broccoli with meat—and I love it.
Your son does need to be careful with pastas, breads, potatoes, and even pizza. A slice or two of pizza can be OK. But if he's inclined to eat 8 slices at a time, that's way too much carbs. Start reading labels and measuring foods and try to keep your sons total carbs per meal to around 45 to 60 grams.
And desserts need to be a once-in-a-while treat, not a regular part of the meals. It's easy to get a whole meal's worth of carbs in one dessert serving.
Finally, what does your son do for exercise? Is he interested in sports? Skateboarding? Hiking? Bike riding? It is essential to help him increase his physical activity.
I hope these ideas help a little.