Here is a list of books to help with teaching the alphabet:
http://www.education.com/reference/article/Ref_Alphabet_Adventures/
 
There are also some alphabet-related preschool activities that your son might find fun:
 
Lace the ABC's
http://www.education.com/activity/article/lacetheabcs_preschool/
 
Go on an Alphabet Photo Shoot!
http://www.education.com/activity/article/alphabetphoto_preschool/
 
Bake Alphabet Cookies
http://www.education.com/activity/article/alphabetcookies_preschool/
 
Turn Your Child into a Letter Detective!
http://www.education.com/activity/article/alphabethunt_preschool/
 
I hope you find the above resources helpful and effective for your son's learning of the ABC's.

Check out this article on using magnetic fridge letters for practicing ABC's. It has a bunch of fun ideas for different ways of using those little letters!

There is a wonderful program offered by ACE Ministries Called ABC's with Ace and Christy reading readiness program. This program combines songs for each letter and sound, workbooks, Parent's Manual, games, stories, Cd's, the works. I have used this program myself and it is phenomenal. My child was reading on a first grade level at age four and she loved every minute of it.

Sing the ABC children's song to him as you show corresponding flash cards. After he gets a little proficiency in learning it have him pick up the proper flash cards as you sing it.

I would suggest trying a multi-sensory approach.  Using all of the senses gives many options for the information to be processed by the brain.  In addition to the sense of sight in association with each letter, use physical activities (ie:walk the lines of a giant letter A written on the driveway), the sense of taste (eat an apricot), the sense of touch ( use applesauce to fingerpaint the letter A), the sense of smell (smell an anchovy) etc.. Novelty will also help with the connections.

There is a program/curriculum called "The Letter People"...its an oldie but goody :)  You get blow up letters and wacky taffy songs to go with it.

First, how old is your son?  Are you teaching letter identification or writing of the letters?  There is a definite developmental progression for learning and writing of the letters.  For example, diagonal perception does not come in until between 4 1/2 and 5 years of age.  That's why many letters such as A, K, M, N are hard to write for many pre k children.  So make sure he is developmentally ready for learning the abc's by consulting books such as My Four Year Old by Illg and Ames or My Three Year Old by the same authors.

I bought a Leapfrog learning dvd set for my 4 yr old son. My son loves it, now he knows the abc's and the letter sounds...Leapfrog learning dvd is  great.

These are all very good ideas.  I have to add my favorite to the list: Sesame Street.  They have a long list of videos on their website.  See the link below.

I would like to second the suggestions about LeapFrog DVDs and refrigerator magnets. If you are not opposed to having your child watch DVDs, these would be a good choice. "Letter Factory" introduces all the letters and their sounds (for the vowels, they just do short vowel sounds on this DVD, but later ones deal with long vowels, as well as blends, etc.). The letters are given personalities and catchy tunes. My kids both loved the DVD. They learned the sounds of letters very young and were able to retain the the information because of the supercatchy tunes.
 
LeapFrog also makes a "Fridge Phonics" alphabet set toy that plays the same tunes as on the DVD and provides a tactile interaction with the letters. But any letters children can hold and touch would work well (refrigerator magnets, wooden alphabet puzzles where the pieces are shaped like letters, etc.).
 
I'm not sure how appealing these would be if your child is older than maybe 4 or 5, but for younger children they are great.