Just write it directly to the officer. Put your info (including address) and maybe even say he'll stay with you. Also put yourself on the hook by saying you're responsible for his money (essentially give them your info coz they feel they can prosecute you if he overstays -- think medieval vouching).
I've read here that they should be finanically independent from you, though every time I invited family I said I'll be responsible for fees (and sent my bank statements to them to attach wit hthe application). Make sure they show their own (healthy) bank statements though.