If your snake is 8 months old he can easily take an adult mouse or a rat pup. (they are the same size)
Get a prey the size of the girth of your snake (wides part of your BP's body)
Mouse sizes chart
http://rodentpro.com/catalog.asp?pro...el=frozen_mice
Rat sizes chart
http://rodentpro.com/catalog.asp?pro...el=frozen_rats