Potty training help

I have a 7 year old son, a 6 year old daughter and a 3 year old son who I'm trying to potty train.

The last month has been doing good. The problem is he will use the potty without any problems as long as he is running around naked. We Dont even have to remind him he just goes when he needs to. However, if I put underwear (loose fitting), a pullup or even pants on him with nothing underneath he won't even try to use the potty even with us reminding him.

I have no problem letting him run around the house naked but we have taught our daughter not to look at boys naked (and taught the boys not to look at girls) so now I can't really let him run around naked with her here but I can't send her away until he gets the hang of it with clothes either.

Right now he is naked and uses the potty while she is at school and then he doesn't when she's home because has not naked.

How do I get him to use the potty without being butt naked?