Well, here is a pretty simple solution for your problem from the experience I have achieved in my English Teaching Carrier.
First of all check whether your child knows and understands the sound of the letters or phonics. If not, make him to learn the sounds.
If your child knows the sounds, again you make him learn the spelling by recognizing the sounds only; do not make him to learn the spelling letter by letter.

Make it multisensory.  You can use the following techniques.
 
1.   Air Spelling:  Choose a word.  With their finger, students spell the word in the air and say the letters aloud.  Tell students they must be able to "see" the letters as they are written in the air.  When the students get to the last letter, they underline the whole word as they say the word aloud.  Ask them if they can "see" the word in the air.  After their response, ask these type of questions:
- what is the third letter? - what is the last letter? - can you spell it backwards?  Remind the student that the word should be floating in the air in front of them and that they must continue to look at it throughout the activity.
 
2.  Stairsteps:  Write the words as if they are stairs, adding one letter each time.
S
SP
SPE
SPEL
SPELL
 
3.  Rainbow Words:  Student writes a word with his/her pencil.  Then traces around the outside with a crayon, hugging the same but not touching the letters.  Student can then pick two other crayons to continue tracing outward.
 
Does your son have difficulties with reading as well?
If a student has a learning disability, such as dyslexia and/or ADHD they must learn in a multisensory way.  Multisensory means using all of your learning modalities.  See it, Say it, Hear it, Move with it.  Often a child with dyslexia - memorizes words or makes visual pictures out of words.  Because of this they sometimes have difficulties with the little words like "and, the, of, etc." because these are harder to make visual pictures of.  Check out the following warning signs and symptoms of dyslexia, to see if your son might possibly fall into this category.