Standby for the uniform police to interject, but here’s my advice!
The RAF don’t really do uniform to fit genetic deviances is this nature, so I would adapt and overcome!
If you have a jumper to fit the guy, butcher a Size 41 shirt so the collar shows fine - chop the arms and bottom bits off. Wear the size 46 shirt underneath with the top bits of the size 41 shirt on top, then his jumper. He’ll look fine and the parade will go swimmingly!
Alternatively, go and buy a shirt from M&S or wherever that does fit him. Take the financial hit from school coffers. It’s not like there aren’t a million different shades of light blue shirts already in the supply system. Ignore the fact it doesn’t have epaulettes - is he really going jumperless for Remembrance Parade?
Get him to enjoy his Tutor experiences whilst they last.
And well done you for getting enough other uniform to fit the lad!