I would be honest, the lying makes it worse imo. Especially as he is autistic, he has enough to worry about.
Anti semitic abuse is fucking unacceptable. I agree with you that in London secondary schools, he will be asked where he is from, without question, often by other ethnic minority boys tbh. Lots of londoners take their heritage very seriously.
And I agree that, at the very least, low level antisemitism was rife among boys when I was at a London secondary school, albeit years ago. 'Jew' being used in the same way that 'gay' might be used.
I think the best way to approach it is to tell him to be honest, and then tell him how to handle push back with preprepared comments.
'Yes I am Jewish, but i am not part of the IDF, I am 11'.
'Yes, I have Israeli heritage but please, I dont know anything about politics, I'm just a normal London kid - i like the xbox'
A younger person will be along to help with phrasing, I'm sure 🙈
Hope that helps. Yanbu to want to protect him, but i think that, especially given his autism, he should be upfront. And you can approach the school to ask them to keep a quiet eye out for him on that front. ❤️
Edit - i hope i haven't minimised things. I understand that you may be afraid of violence so dont blame you for wanting to prevent that. I just think its going to come out.