There is no need to apologise for coming back - we are here to help.
If he fulfils the 4 criteria, then you should tick the box. A consultant's letter will suffice as evidence - I cannot imagine for one moment that she will not state how it can affect his daily life.
You can still use the disorder/disability (and the diagnosis so soon near to the tests) as mitigating circumstances. The 11+ is not a "one size fits all" process, and a child with Tourette's might well not respond to it in the way another, non-disabled child, would do.
As the diagnosis was made only on the 30th September, it would have been nearly impossible to assess what his needs were in time for the first test, and make special arrangements. The Head did allow him to sit the second test separately, so to an extent that was a "special arrangement". Whether extra time would have been granted to allow for his tics, no-one will ever know.
I feel that you - and the Head - did all that you could in the short time span available, and I think that the panel will recognise that. If anyone should have spoken to Admissions in those 10 days, it was probably the Head rather than you. However, I feel that you - and the Head - did all that you could in the short time span available, and I think that the panel will recognise that.
Admissions do not grant special arrangements without considerable evidence and deliberation, so I doubt that either of you would have got a result in that time frame. In the meantime of course, the exam stress was building up in your son, and delaying the test would probably have made that even worse.
When the LEA rep makes his statement he will say to you words to the effect of "I understand that your son has a disability - Tourette's syndrome?", and you will simply need to say "yes". The time for discussion is later. If he does not mention it (which is very unlikely), you might say at the end of his statement: "I would like to add that my son has a disability, TS, and you will all know from our statement that it is a significant part of our case."
You might like to consider finding a "one page summary" of Tourette's to include with your evidence, "that the panel may find helpful if they are not familiar with all aspects of the disorder". Tourette's is only known to most people because of the vocal tics and involuntary swearing, and there is, as you know, much more to it than that.
I suspect that this will be one of the very rare cases (they are as rare as hen's teeth in fact!) where you may need to go to 2 pages for your written statement, rather than one. Definitely no more than two though!