In general, lamés (electric jackets) sizes are not specified as xl or xxl. The manufacturers will usually cut the lamés to specific sizes and give them numbers like 42 or 48 or 52. It varies greatly from company to company. There are many online websites that sell lamés, but my favorite company is
www.thefencingpost.com . You should order by phone, and if you mention the website, they will give you a 20% discount. They sell a brand of lamé called "JL," which I highly suggest due to its value per dollar ratio. With the discount, it comes out to be $52 (plus S&H). Take your son's measurements, and give those measurements to the person on the phone. I'm sure they will be able to find something for him.
Also, I think it is important to keep in mind that fencing alone is not a very effective way to lose weight. It is a very fun physical activity that will defiinitely help, but in addition to fencing, perhaps you can convince him to perform regular cardiovascular exercises (to help his heart and stamina) and weight lifting (to increase metabolism and confidence).