R.K.
My 12-year-old has a severe allergy to sesame seeds. The best defense is to teach your child to be watchful himself. After a few reactions, my daughter is very careful to inspect food and read ingredient panels - or make me read them if I forget. Your step son's allergies are going to be much more difficult. But a little homework in advance - lots of reading of ingredient panels will help you know for sure what products you can buy and what you can't. Most foods now have an allergen alert at the bottom of the ingredient panel, making it easy to find the most common allergens easily.
A warning about sesame seeds - they are not always visible! When you order burgers from a restaurant ask for a bun without sesame seeds. They may offer you the bottom of two buns instead because only the tops have sesame seeds. You or your son then have to closely inspect the bottoms because there will be stray seeds cammouflaged in the bun. I always do this inspection for her so she doesn't end up touching them. Also, if you find a brand of bread you like and one bread in their line has sesame seeds, there's a good chance that a random seed could end up on a non-sesame product. Make sure others who are eating with you keep their sesame seeds away from your son's space.
Crackers - you'd think the sesame seeds would be obvious, but some brands have the sesame seeds crunched up and sprinkled on top. Never trust any cracker or bread product no matter how tame it looks - ALWAYS read the ingredients!!
Hummis - we just ended up in the ER a few weeks ago over this. We never eat hummis, so had no idea that it's made with tahini, which is basically sesame paste (like peanut butter.) My daughter took one taste of it and her lips swelled and throat closed. So any food that has tahini in it is concentrated sesame seeds. That's a really bad thing.
Other than learning the good foods from the evil ones, always keep Benedryl at home, in your purse, in the glove compartment of the car, at school...wherever he hangs out. That stuff works very quickly.
Good luck!!