DD is 2.5 and has been loving shape-tracing "worksheets", she keeps asking me to print more.
She knows all her letters, most of the sounds, and has even started sounding out simple words! She can count 'by rote' to 20 and does one-to-one counting and manipulation (subtracting one, etc) accurately up to 5. Knows all her colours and shapes. Can draw a (mostly) recognizable person, with a face, legs and feet, and arms.
She learned ALL that from observation and from self-play with manipulatives. NOT from worksheets. And with extremely little 'teaching' from us.
So basically what I'm saying is... if he really does like worksheets, have fun with them. My DD is certainly asking for more. But don't FOCUS on making those the MAIN EMPHASIS of your "school time". In fact, at this age, 'educational' stuff really should be more about 'practical skills' -- pouring water from one container to another. Fastening buttons and zippers. Spooning marbles from a big cup sorted into little cups. Walking on a curvy line on the floor. Helping with cooking and washing.
We've done a ton of this with DD, and she's loved every bit of it. As a result, she's very confident and independent. She loves to do dishes (and does a half-decent job too lol), sweep the floor, sort and fold laundry, cook hot dogs, crack eggs... Her very favourite activity right now is spooning marbles... She's also enjoying arranging silk flowers... Cutting paper and playing with stickers... She'll go off into her room, set up an activity, and engage in it for like a half hour, entirely by herself and on her own initiative.
All of these are, in my opinion and that of many great early childhood educators (esp. Maria Montessori), MORE important for their mental and intellectual development than the "pure" academics. It's what they're itching to learn right now, it's what they'r designed and primed to learn right now.
There's even some research, so I've heard, that this kind of fine-motor practice actually improves the development of the frontal lobes -- which become all-important in logic, rational thought, intelligence, etc. I've just heard this second-hand, haven't seen the research myself, but it's intriguing. I am CERTAIN that our daughter's intelligence and curiosity and obvious skill with "academics" has only been helped by our focus on "practical" skills over those "academics".
So anyway... I have a DD who seems to be the type who will enjoy worksheets, as I know I was as a child. DS was (and still is) NOT that type, and I know in hindsight that I seriously damaged both our educational relationship and his passion for learning by forcefeeding him worksheets too early, and too often.
So just be careful with them -- they can certainly be part
of a preschool curriculum for a child that enjoys them. But don't make it the only thing, or even the MAIN thing for your 'school time', certainly not at this age. Give him more physical stuff to do and you will be greatly rewarded for it.