With regards to the bedwetting alone, I don't think it would be right to ask the mother to oversee that your son uses an adult nappy or to warn her to use plastic sheets, that would be humiliating and embarrassing for him which could trigger a bedwetting session anyway. At his age he will still fit perfectly well into junior plus 'drynights' pull-ups which he can discreetly put on in the toilet or bathroom when he changes into his nightclothes and he can discreetly bin them in the morning, or wrap them in a carrier bag and put in his overnight bag to throw away at home. No fuss or problem there at all, no need for anyone to know about his perfectly natural occasional accidents. As for whether he should go at all or not, no one can really say yes or no that's your decision but if it were me I would simply telephone his friend's mum before had to ask about the stuff you raised like movies etc. We recently moved to a new area and our sons have just made new friends with boys whose parents we don't know yet so I know exactly what the concerns are.
Melissa.

I agree with Melissa about the bed wetting...about the sleep over itself, If you’re uncomfortable I wouldn’t let him go. If you’re just over protective I would give it a try...it may be good for you both.
Go over for a hr or so when the party starts instead of dropping off your child and leaving....I feel the same way about my child