Boys often like a more practical reason to be mark-making.
So if he likes cars/transport:
If he enjoys construction: clipboards with pens & paper (just as a foreman would have) notebooks, diary, post-its. As part of the normal 'construction' play equipment.
Being a 'helper' with your every day jobs- if you write a shopping list, he can 'write' one to. Doesn't matter what marks he makes. He can even draw pictures of his favourite fruit that he wants to buy. Maybe you play eye-spy when catching the bus next time. You each have a notebook & make a tally mark for every blue car you see. He might choose his favourite colour- who will see the most? This way he sees you role model the importance & various uses of writing and he will begin to understand the various concepts of literacy. It would be great if he sees you writing letters, crosswords, postcards, post its. Anything!
Writing letters to people that will handwrite back- Nanas, cousins etc... Include the trip to the post office, paying for stamps.
In the bathtub or on a warm day outside- paintbrushes & water, or a little colour/paint in the water. Add chalk (pavement chalk is best if working outdoors on concrete) so he can make marks with this first, then wash it away with the wet paintbrushes.
Make your own books for bed/story time- give him nice paper, stapler (to learn to use himself) or hole punch & ribbon. He might like to dictate the story to you. A prompt for this activity might be reading one of his favourite stories- then posing a few what ifs... For potential alternative endings. What would have happened if?
I wonder how...? Try not to dictate to many rules for his creativity here, give the prompts then just listen. If his story if 5 words long, after you read this, the next time you make a book it might be 10 words.
To encourage him: Give only specific praise (as opposed to generalised "good boy" etc...) If he spends longer than usual writing praise his effort. If he forms nice lines & circles-give specific feedback on that. Praise effort, praise concentration & trying his best. Try to remove wording like "writing properly" or doing it right. At this age it is about experimentation, trying his best, making mistakes is an important part of learning. We all make them! So effort is more important than accuracy.