I'm glad that you won't be putting him in a kennel anyway, I personally don't like Cav's in a kennel, they are an extremely friendly, loving breed, hopefully he has not been badly treated or is badly bred as a breed he should love children, dogs, well everyone, Cav's are like that. :-)
You will just have to train him as a pup, he will soon learn to toilet outside and will take to living indoors like a duck to water and having other Cav's as his tutors will make his learning so much quicker, he will very soon follow them with regards to where to toilet etc.
Your other Cav's should accept him no problems and they are only small dogs so one more won't make a lot of difference,

it would be great for this dog to have other friends and family, and bless to be indoors.
I say if your heart is in it go and rescue him and give him a good, loving home. :-)