A six year old doesn't talk? Are you a therapist or his teacher? I don't know what therapists tell you to do about that. I'm sure they have a plan. Just as a mother/neighbor, whatever reason I would have to be around a child I would try things, I would try to see if he could use a camera to show you things. I don't understand why you want him to write if he doesn't talk. Are you trying to build fine motor skills, or get him to communicate?
He can only write if you grab his hand and write with him? He doesn't talk? So why are you helping him write the alphabet? I'd rather see him paint pictures that represent objects such as people or trees. Teach him how to do a stick man using a paintbrush. Draw a circle and teach him to draw the generic Smiley inside. The written word, how can I say this? For example a new reader will sometimes get stumped by a word if he knows the meaning, but a word he doesn't have the vocabulary for looks like gibberish. I would not neccessarily start with the alphabet or written word. Then you traditionally have a year or two of "creative, phoenetic" spelling. Written communication takes a lot of work and a long time.
Why are you calling him gifted and with the Einstein Syndrome. What does he do that lets you know he's really "there". Is it some talent he's displayed? Is it his interpersonal relationships? Typing out his Strengths would help someone better try to expand communication with him.
I would try a bunch if stuff. If you want to know what other people would try to do then try to describe the kid now and tell us what you're trying so far.
On the subject of writing, the first step is a game called "Follow Me". Buy the Kumon book called Tracing (it's not really tracing) from Barnes and Nobles or online. Give the kid a fat marker. Put the marker in his hand at the beginning of the excercise. Tap the paper in front of the marker with your finger. Teasing him playfully slide your finger away from the marker, then tap by the marker and slide again until he follows your finger with the marker. Finish the excercise and put it away quickly. You may have to stop several times in the book and wait a few weeks or months because the lessons get complicated as you go on. Celebrate Success. If it's close enough or partially correct- it is correct! (only, if it's partially correct a few times it's time to put it away for a little while). You still cheer, and celebrate, and tell them how good they did.
Coloring, use it more like it is marking, circling, or underlining. In a coloring book, say, "color his eyes". The correct answer is to scribble over one eye, or prefferably both. Color his mouth. Color his shoes. The correct answer is to scribble a little bit over each of these things. Later on they'll underline the correct word or circle the right letter. Right now a little scribble over the picture means the same thing. You marked the answer. You communicated in "writing" (?).
I'm sure you have or can get cheap phonics workbooks. Use the pages where they have rows of tiny pictures (usually asking what letter does stuff start with). Pick a few things on the page and say, "color the bear. Color the car. Color the doll. Color the fox." Good. Put it away. The correct answer is to scribble a little bit over each thing you named.
Now a great reason for taking his hand and "writing" is to get him to scribble in circles. You're eventual goal with this is to form O's, then eventually C's. The first shape you're going for is scribble, scribble, scribble in a round shape. After they do that automatically you should demonstrate a circle and then reach in when they're doing a scribble circle and pick their hand and pen off the paper when it's just a circle and say "Stop". "Draw a circle." "Stop". I would not start with the alphabet until they can write a c.
But my only experience is teaching my toddlers who have great fine motor skills. I still think it's a reasonable method for almost any beginner.
