My litter of kittens were abused by my brother's friend (he is 10 and is well aware of what he does) and my litter became from very active to leaning against the wall being curled up into little balls, what can I do to help them gain confidence again?

If they've been badly physically abused .. I would be more worried about injuries than anything.  They may be badly injured and THAT is why they are acting the way they are.  They need to be checked out .. Immediately if they are suffering.  They could have internal injuries or even broken bones .. Who knows???

Allowing abuse to happen to animals is NOT ok ... Your mom should be made aware as to how these animals are behaving and get them some help for what SHE allowed to happen to them. 

These animals not only need medical help but they need to be protected from ANYONE who would potentially hurt them. There is a lot of responsibility involved when you have such small critters in your household ..  It is your job to protect them from this kind of thing is you and your family's responsibility. Don't EVER let that happen again.

AND .. THIS 10 year old 'friend' needs to learn that this is unacceptable .. He's obviously got problems of his own .. and that is something his own parents should be dealing with.

You need to ask your parents to contact the boy's family.  There is a well-documented relationship between abuse of animals and later serious criminal behavior.  A vet needs to document their injuries (if any).  If they have been harmed, I think you need to contact the police.  This boy may really need some intervention.