Welcome to our forum. Unfortunately, the only cat that is truly hypoallergenic is the Sphinx. The allergy in humans stem from the dander of the cat - not necessarily the fur. Bengals have a pelt, rather than a typical cat coat. And, cats are different. Your son may not be allergic to a particular bengal, but another bengal could trigger an attack. You might want to check out this website and scroll down to find breeders that may be near you:
https://bengalcatclub.com/bengal-cat-breeders/Many will allow visits, but usually for people who have put down a deposit on a kitten already or are ready to select a kitten at that time. Holding a bengal kitten for a short time may not trigger anything, but bringing one into your home full time could. There are medications for allergies that might help your son if you are truly set on getting a cat.
Note, there are many other breeders who are not on that list. I'm guessing you are not in the US, but the UK. You can google breeders in the UK and see if there is one close to where you will be living next month.