I agree with the other moms. Other skills I can think of that they will start working with your son on are "pre-writing skills". You can have him use blunt scissors to cut playdough, trace dotted lines, squiggles, shapes. Work on having him hold his writing utensil correctly, etc.
Just a short tidbit-- some kids are great at sitting down and working on "worksheets" (tracing letters or shapes, etc.) at this age, while others may not be. My childcare center introduced this activity in the room when my son had first turned three, and he absolutely hated it. They had two or three kids work on their worksheets at a time while the others got free play or outside time. My son reacted by KICKING HIS TEACHER!!! (Never before and never since has he reacted so violently to being told what he has to do-- he is really not a rebel). I had a discussion with the director about whether this activity was age-appropriate, and after a few months, they decided it was not appropriate in that classroom.
As he got closer to 4, he really began to love to sit down and color and write letters. He's now 4 yrs, 3 months and actually stays in the lines when he colors! As you know, they all go at their own pace, and it's just really important to let him choose what he wants to do.
Good luck! Your son is doing great already.