I would encourage it. But not by formally sitting him down to do material from the EY curriculum.
An abacus, tangram puzzles, izzi tiles, cuisinaire rods, wooden building blocks, Lego and duplo, little sets of lego-like bits that he can put together would all be nice for him.
You could make little cards with arrangements of Lego blocks drawn on them that he can copy using the blocks. I've seen some ideas online that I can link later.
If you want to encourage writing, get a chalkboard easel and plenty of chalk. It's a wonderful sensory experience that requires hand eye coordination and fine motor skills. For writing on paper, use coloured pencils and crayons instead of markers. Again, better sensory experience, and you have to push the crayon or pencil harder.
Also Play-Doh or modelling clay.
Another good writing aid is a megasketcher or other sturdy erasable slate.
A sand table is great too for making letters, trails.
Look up Montessori writing and fine motor skills development ideas.