I'm not suggesting for one moment that is a guaranteed outcome...just that I have seen, time and time again, how ongoing school refusal can start with one incident/exception and become habitual.
No-one on this thread, besides the OP is in a position to make that judgement call.
It's great that the lad is getting good attendance and is academically able, but if he hates school I would be questioning why. I would be concerned that maybe other kids have said things about his dad and that was why he didn't like school and specifically didn't feel able to go in on the anniversary.
It's more than likely if it's the first year he's had to go into school on the day he's feeling anxious about it...again why? Does he feel he'll be bullied if he gets upset, does he not have a trusted adult at school he can confide in if needs be?
Is it more that he needs the day to grieve or that he's simply feeling anxious at being in a different environment on the day for the first time...which is understandable, but sometimes we have to try and push through those feelings.
Taking the professional hat off (16 years experience working with kids and families, qualifications in CYP mental health and counselling and halfway through a degree in child development and psychology so you'll forgive me if I don't use your inverted commas)...
...if this was my child, I would ask them what they wanted to do to remember their dad out of school time that day and gently encourage them to try and go in, maybe with a lunchtime pick if that helped to facilitate what they chose to do in remembrance. I'd make the relevant people at school aware...and if it came to the day and they felt they really couldn't manage it, then I'd let them stay at home. I simply wouldn't put in place an automatic fait accompli a week in advance.