Did not take the time to read everyone's suggestions as of yet this evening, so this may be repeat, but thought I would respond anyways.
Singing the alphabet song every morning and night before bedtime seems to sink in quicker and they dream about it and will remember it even without much effort. Whatever we listen to or think about right before bed will always affect our subconsious quicker than anything else. That is why it is so effective to pray, listen to music, or read before going to bed AND first thing in the morning when our minds are clear and uncluttered from the day.
Watch more video/dvd teaching the letters with their sounds and songs, songs just click with MOST children...especially with the ones with delayed mental impulses. Music/Songs to anything are easier to commit to memory. Anything he does WITH you will be more effective. Kids learn more, heck, everyone learns more when they are having fun! Come up with your own little dance to the first numbers 1-10... Make up a little rhyme of your own.. 1,2 put on your shoes; 3/4 go out the door, 5/6 find some sticks...7/8 dont' be late 9/10 to do it again...Do this to a chalk hopscotch outside...
Or sing the original one.
Leap Frog dvds/videos and. cd/tapes are usually available at the public library or home school trade centers. You can listen to them in the car/van all day long on every errand or anytime you are in the car/van. You can also rent little travel dvd players I think if you don't already own one. Or maybe borrow one from a friend. Check with your local preschools to see what they might say about what works best as far as memorizing letters/numbers.
Get an easy wipe off board and write a letter or two every week and talk about letters constantly. Ask , don't just give the answers.
Put their name on every item you can in their room with a label to help them recognize the letters in their name
Label items in your home with the name of that item with the first letter a bit more identifiable. Shelf, Bed, Chair, Sofa, etc etc. Much like you could/would do if you were learning a foreign language yourself :O).
Treat letters like a foreign language.
As for numbers.Count everything in sight. Every few moments throughout the day. Count the seconds it takes as you dress him. Count the number of buttons he has on a shirt or the things he may put on in the morning. How many plates on a table as he helps you set the table for dinner. Count how many steps you are taking... Count and sing the numbers to your phone number or any sequence you want him to commit to memory as well. They only have to count to 10 for Kgtn, but then need to try to get to I think 20-25 by first quarter of kgtn. Count his fingers and his toes after the singing of the alphabet at night by ritual ...
IF he has learned anything that you as a family do by ritual, he can learn the alphabet and numbers, no sweat.
Ask him to recite numbers 1-10 before every snack or thing he asked you for. Make a trade off, if it is just a stubborness.... this worked with my little headstrong 4yr old who refused to learn our phone number.
This took one day and no snack was "free" :O)
Teach this all with your 2yr old as well, it is NEVER too early to learn this ...plus they are still at the age where they WANT to please Mommy .....