I know of a free website called Starfall.com. This site helps with learning letters and sounds. Make sure you visit the website first and play some of the games. Then, sit with your child and play the games together for fun.
If your little one needs more practice with letter naming - start with a game that helps with just that skill. On the site, look for an icon that says "ABC".
If your little one knows his letters but not sounds - then find a game on that website that works on just that skill.
If he knows letter names and sounds - then find a game that works on putting sounds together to make a word - - EASY words like c-a-t - -words that have two or three sounds ONLY. Once he gets good at that, start working with words that have more sounds. STAY AWAY from words that have silent letters in it - like rake - it has a silent e at the end of the word. Start easy and slow - lots of practice and patience!!
Also, read EVERY night to your son. If you can find easy reader books - a book that has easy text and have one sentence on each page - be sure the "picture matches the word" (that way, if your little guy comes to a word he doesn't know, he can look at the picture to help him figure out the word) - buy it and read it each night with him. Have him point to the words with his index finger as you read.
If he's in school, request a conference with your child's teacher. Find out what he's having trouble with, and ask for some ways you can help at home.
I'm a former Kindergarten teacher, can't you tell???
Good Luck!!!
M. :)