I'm not sure what everyone has answered, but I will say I do NOT like Hooked on Phonics, My Baby Can Read, or anything else like this.
However, I homeschool, and I started my youngest when she was 2 and a half on pre-K stuff, and you DON'T have to spend a ton. Here's what I did.
First, we used a Magic Doodle (or Doodle Pro, or whatever...like, the pen that writes on the board and you swipe across with a slider to erase it) to learn letters and numbers. Start with the capitals, because that's what they need first. After he is comfortable with all of the capitals and can recognize them, write them when they are said, and tell you what sound each one makes, then you can add the lower case and they will come easier than if you try to do them along with all the first. Pick a very common picture to represent each letter...apple for a, ball for b...and sound them out, like aaaaa-pple, b b b all, so he understands letter sounds and knows what pic to think of to remember. Also, make sure you teach the short vowel sounds...apple, elephant, igloo, octopus, umbrella...and not the long vowels. He will eventually learn that long vowel sounds are just the letters.
For handwriting, I absolutely LOVE the Handwriting Without Tears program. Very inexpensive, and WONDERFUL...I wouldn't recommend buying the teaching guides, I would just go with the workbooks and move along in them. My daughter could write fine at 4, but when she started the HWT program, I swear, her handwriting improved SO much that now at almost 6, she writes better than most adults I know. And that's no joke, that's how great this program is.
You can buy inexpensive "pre-K" workbooks in the book section at Target (or probably a lot of other stores, plus Barnes & Noble...) and just look for one that has colorful pages. They have a variety of activities...counting, letters, simple addition, shapes, colors, etc...and they can be a lot of fun.
I tried the Kumon series for some pre-K stuff, too, and my daughter didn't enjoy them much because they weren't much fun. There was a "scissor" book, or cutting, or something by them that she did enjoy, but the more educational ones are a bit of a bummer.
Also, games can teach SO much. Play Monopoly, Yahtzee, card games, Life, dominoes, etc...really, they reinforce a lot of counting and adding skills.
I went through a TON of different workbooks from the B&N educational workbook section, and I can't even say which they were anymore but we liked all of them. Just really check them out, and see which ones you think would be fun.
Lastly, if you are going to do this, I would do it very regularly, but not for any extended time. I think we did about an hour a day, 3 or 4 days a week, and that was enough. Play is important too, because soon he'll only have weekends and summers!