If he is very interested in makes sense to support it, it just all depends on how you do it as to whether it is advisable or not. It is a very sensitive time and the priority needs to stay on the enjoyment not proficiency at this age. There is plenty of time to develop skill, but it is very hard to regain motivation if learning to read becomes a chore.
if you are talking about looking for brief teachable moments, how you answer his questions, how you extend the activities he is already choosing, then great. If you are envisioning sitting him down to 'teach him'. That is not so great. Keep it brief, fun & child led.
You say he knows his letters- does he know his letter sounds or just their name? Start calling the letters by their sound.
Make sure you are role modelling the sounds correctly e.g for M it is mmmmm not 'mah'. Cbeebies Alphablocks can demonstrate the correct sounds ( www.bbc.co.uk/cbeebies/alphablocks/watch/alphablocks-watch )
What about rhyme? Can he identify rhyme in a sentence & give his own examples?
These would be the places I would start.
I would recommend visits to the library- regularly choosing a wide range of books, non-fiction, reference, rhyming, song, poetry, fiction.
The wider your child's vocabulary- the easier it will be for him to become literate.
Look for teachable moments- when he is asking you "what does this say?"
Help him use his developing letter sound knowledge to guess, sounding out the words. Emphasis that it is always better to try & guess, rather than getting it right (this applies to most things at this age)
Same goes with when he is trying to write something. "I wonder what it might start with? " Focus first on initial letter sounds.
Middleagedmotheroftwo correctly points out the next stage :) - CVC words. (consonant, vowel, consonant) There are plenty of fun ways to introduce those once you have mastered the letter sounds & rhyming. I would exercise caution introducing these in any formal way (flash cards, word tins etc...) That is heading down the route of too formal too early. Update us more specifically on his capabilities & interests and I can help you develop something more fun & child friendly :)