You can put any date you like, you call the shots, but we found it best to let DD have time to say goodbye to her friends & teachers, sort of closure, rather than just disappearing.
Her HT thankfully was great & allowed her part days school for a week, which meant she didn't have to be in class with the teacher who was demoralising & stressing her out so much, she made a cake in school to share out & the kids made her a goodbye card, so she left on a high, rather than skulking away deflated & defeated - if this is possible to arrange similar for your DS, then I would recommend it.
teacher that's really good to hear that your DS was fine in his next school, this is something that worried me as DD, though a toughy & faces it anyway, she has become very mistrustful of girls her age & visibly backed against the wall visiting the yr6 class in her new school, yet walking confidently & bubbly into every other class they showed her.
Glad your son has benefitted health wise too - after dealing with daily IBS flare ups, exhausted by her diagnosed EDS, helped along by stress & not sleeping barely at all, depressive episodes & nasty eczema flare ups that kept getting infected, daily headache, to list just a few things -
Since finishing school, mine has slept well, had one minor IBS flare ups, the eczema has gone completely & one minor headache & is her old bubbly self again :) - so sad it can affect them so badly when in DDs case, she was considered a "model pupil" & just lives to learn, as a lot of our DCs probably are & I hate that it came to this just so wrong at this stage of her education, but something had to give & I was going to let it be her health & well being :(