We're big fans of www.starfall.com and www.noggin.com . If you explore both of those, you're sure to find something good on his level (from Noggin, I think the Oobi games are especially helpful--click on the hand character with the eyes).
We're also big fans of "cheap"!
Early on, I tried to get my daughter interested in writing-practice books, but it was hard to motivate her to do much more than draw in them--those just weren't the things she was interested in writing! Then I came up with a great trick. Let your son draw and tell you the drawings' titles (including "by....."), or make up his own "books" (folded paper), or entice him to make signs labeling things around the house--maybe even set up a scavenger hunt for Daddy with simple signs and arrows. In other words, find out what HE wants to write! Then, whatever he's excited about, write it for him yourself, boldly and clearly, with a highlighter or other light-colored marker, saying the letters as you go. Afterward, ask him to pick up a darker-colored marker or crayon and trace over your letters to make them readable. The added bonus here is that he learns the pride and power of getting HIS ideas in print!
A friend of mine got some other great ideas while researching Montessori methods. As others pointed out, electronic buttons only let your youngster look at the letters most of the time. What she discovered from Montessori was that if you cut out large letters from fine sandpaper or soft furry material--different textures that are really fascinating for them to touch--and glue each letter onto its own sturdy card, the child can trace the letters with their fingers over and over again and really learn them quickly. Show them first by moving their fingers in the order you'd do it to write the letter, and of course say the letter out loud as you do.
Another Montessori trick she discovered is to go to Michael's or some other craft store, and find the little wooden cutouts they have of various objects (they're small, sturdy, lightweight, and very cheap) or make use of baby-puzzle pieces you already have at home, or even pictures of family members or pets. For each one, write the word for that object/person on a separate sturdy piece of cardpaper. Read through all of them and show him the objects/pictures, then lay out a few and let him match them up. The great thing about this game is that it's self-correcting; if he makes a mistake, he'll figure it out himself when a word that he does recognize no longer has its match available. Don't worry about how long the words are. My friend made a large set of these for her 4-yr-old son, and to her surprise, her 2-yr-old started playing with them and correctly identified "pineapple" and "lighthouse". Until they start actually sounding things out, it's all a matter of recognition--and word length is a clue, too--but that recognition leads to paying more attention to individual letter sounds. It also makes errands more exciting, as your son starts proudly shouting out "pineapple!" when he recognizes the word on a sign at the grocery store.
If you haven't seen them already, the Kumon series of workbooks is really great, too, especially if you also want to give your son more practice in reading numbers. They have a whole display full of them just outside the children's section of Barnes & Noble, for about $5 each. The series is actually from Japan, and they're extremely appealing and well laid-out. My 4-yr-old asks to work on them all the time!
Maybe you can explore websites of Montessori tools as my friend did, and come up with more ideas. These don't have to take more than a few minutes a day. Of course you don't want to put stress on him (or yourself) by forcing him into what you know he'll eventually learn just fine anyway. But let's face it--the best scenario is that he's actually having fun, both with the activities and with the new knowledge he gets to enjoy.
One more note...I don't know your exact situation (especially with another child on the way), but the bit about his being tired after school really hit home. I tried putting my daughter in preschool this year, thinking she'd enjoy it and learn a lot, but it had the opposite effect on her. Instead of pouring herself into these workbooks that she loves so much, all she could do was whine and fuss when she got home. Also, she was rebelling against the structure and lack of individual attention at school, so that she wasn't even showing skills there that she'd had since she was 2 (scribbling instead of drawing, for example). Plus, I was so tired from driving her back and forth that our usual learning adventures and one-on-one time were becoming a thing of the past. I reluctantly pulled her out after 3 weeks, and her voracious appetite for learning came right back. She's now reading longer books by herself, and we're thinking of homeschooling her at least for another year. I don't know if that's a possibility for you, or if it even suits your son's personality and needs, but it's something to consider, especially since Mommy time is going to be a very, very precious commodity after April comes along!