I am very flattered to be the first one to help you.
He is a kid so, some time after the initial shock, he will quickly get into learning new things.
Have a look at this site. It is a bit daunting but there is a lot of material and articles for a much fuller guidance. On the left hand side column there is beginning English which will then link you to many items for you to use:http://esl.about.com/od/beginningenglis ... SL_EFL.htm
Along with general greetings and introductions, you know, "I am Jasminade...", point to him, and he might say his name, repeat I am and then point, this time letting him hear the question "who are you?... What is your name?" You will be pleasantly surprised at how quickly this will cross cultures.
Then the general greetings, "hello, how are you?", using pictures you could show day/night/morning to show the different types of greetings... you can then move on to objects, using pictures.
You remember learning a foreign language? You know, " what is this?" "this is a knife..." Make it fun and a wee challenge for him to guess what it is... And overall speak the words for him to repeat, repeat, repeat... (and associate with the pictures/objects indicated).
Like with all kids they must learn the "look" of the alphabet. Keep it simple and use step by step words. And make all of this relevant. You know, even though he is an Afghan, he must know something about sport? Maybe not, but try to make it relevant to where he is coming from and where he is. Use the TV, use everything. play some popular music and if you spot a song he likes a tune to, download the song, google for the lyrics (if they are not risque), take some words out using word, and get him to guess and copy the sound. Even if he does not know the word it still will be exciting. I am learning Mandarin Chinese, and already a know a few lines of a couple of songs. Songs are invaluable for learning how to pronounce difficult sounds.
Type in flashcards (with tefl, esl or something) in Google and see what comes up.
Next step getting him to learn verbs. The first three important verbs to learn are 'to be', 'to have', and 'to do'. To put it crudely and simplistically for the purposes of this stage of learning, they are about who you/we are, teacher, foster parent etc.; what we have, in our pocket/hand etc. or brown eyes; and what we are doing, teaching, learning, caring... Then you build more verbs using questions, looking at pictures, and of course, vocabulary, including nouns and adjectives.
Just give him the foundations, make it relevant, and watch him fly!
I hope this helps, a bit muddled, but I hope a start.
Oh aye! You are a teacher, so you can always use sites such as this for further advice. We all try to help.
Good luck to you and the boy!