Here is a link, for kids medical safety bracelets you might find useful:
http://www.mypreciouskid.com/medical-id-bracelet.html
YOu best see what the school's policy is regarding this.
My daughter's school for example, has a strict, "no peanuts, no nuts or tree nuts" policy... which means NO ONE can bring to school or serve anything with nuts in it in any way. There are also teacher's there which are "approved" for giving epinephrine shots in emergencies, in case of accidental exposure to the allergenic substance.
You son, per his Doctor, may/should be able to have a Doctor's note and/or Epinephrine shot with him at school at all times, as a precaution. As you know, this can be a serious reaction if it occurs.
If your son's school does not have such a policy in place... they are liable for things, I imagine. Also, it will possibly impact your son.
This will not only affect him in preschool, but in Elementary school as well, and onward. So , it's best to research and look into any school, ahead of time, that he may possibly be attending. ALSO in any public place... always carry your epi-pen.. .you never know who or what or when anyone will have any sesame residue on them or around the premises.
At his age, you can also start to teach him how to say and tell people/teachers that he is allergic to sesame seeds. This will serve him well in the future.
My daughter has an allergy to Kiwi fruit... which is also a highly allergenic food... if consumed. And it brings on the same reactions your son has to sesame seeds, but can also cause swelling in the throat & a prickly pain in the mouth, thus asphixiation. With us, my daughter knows so she does not eat Kiwi... and we notify any school she attends. She can also tell people this herself, since we taught her, if we are not with her. This has worked out for us.
REALLY see what your school's policy is... and yes though, if a child comes from home after ingesting or eating or touching any sesame seeds... it can possibly affect your son. So, it's hard to control what people do at home, before coming to school.
Good luck,
Susan