Mrsbadger - how about loose fitting cotton shirts (no fuss, no iron kind of course!!) or normal t-shirts? No top at all also works for me with little kids when it's hot (boys and girls)...
...with sunscreen of course.... before anyone starts.
izzybiz - get back to us when DS (or DD) is 18 and let us know how that went
Where did you get DS's sleeveless t-shirts from? I just want to have a look at some and can only find 'vests' not nice sleeveless t-shirts.