The original poster on the SEN forum was asking about a diagnosis after the test but before
National Allocation Day.
While it is correct that there is no statutory time limit for submitting an appeal (Appeals Code, para. 2.9), I'm afraid Herman's right that there can be a limit on how long an 11+ qualification is valid for. In the case of the Bucks 11+, it would remain valid until December of year 7.
If your child qualifies but does not or cannot join a Buckinghamshire grammar school, his or her 11+ qualification lapses at the end of December of the year of proposed admission. [Guide for Parents, Secondary Schools]
There are some advantages in going for the late selection test - the standardisation is easier, and generally speaking
scores tend to be higher.
You will have the opportunity this time to request special arrangements, and - in the event of an appeal - the closer your son is to 121, the better your chances. No guarantee that a grammar school place would be available, but you would of course have a right of appeal.
Bucks normally require level 5s in Key Stage 2 English and Maths to resit the test. If your son didn't get level 5s, you would probably have to appeal for entry to the late selection test. There's a good chance a panel would look favourably on such a request when moderate to severe dyslexia has only recently been diagnosed.