Does he have any issues with motor coordination or delayed motor skills? If so look into developmental dyspraxia/developmental coordination disorder.
DS is the same age and started out with normal motor development (for instance he could crawl at 8 months and walk without help at 10-11 months) but plateaued around age 2-3. He had a brain injury that seemed to make things worse and has gotten a lot better, but still has problems with things like hopping, catching a ball, holding a pencil correctly, writing neatly, etc. If the only problem is writing and not other motor skills it could be dysgraphia.
DD has ADHD and her handwriting is slow and she tests slightly low on fine motor coordination tests. But she does well with running, jumping, gymnastics, swimming, etc. It's just manual dexterity or bilateral coordination (in terms of fine motor) that's a bit of a problem, which is common with ADHD. She can write neatly but is really slow.
In your shoes, I would probably ask for a referral to a pediatric OT for an assessment (and possibly PT if you have any concerns about other motor skills). For a diagnosis of DCD you'd probably have to see a neurologist or neuropsych.
