Thankyou for the suggestions.
The tshirt printer thing sounds like genius, zzzzz - and I especially like the idea of putting something on that ds can recognise as his. I think it is going to be a struggle to get ds to wear uniform full stop, but if I can do something like put a skull and crossbones next to his name, it might help.
The trouble I find with sharpies, is that unless you write it on the label, it goes through to the other side on polo shirts.
I have tried sewing labels flat, but he just worries and picks at it constantly.
I'll have a look at the tshirt printer stuff, as I could use that for the girls uniform as well.