Is my 3 1/2 yo behind? He refuses to do workbooks with me, and doesn't want to write.?

My daughter is about the same age. It sounds like these are fine and gross motor skill problems. Or it could be something like ADD or ADHD. They are both attention disorders that will just some hard work and support can be corrected. You can talk to his doctor about seeing an occupational therapist, that should really help him. Hope I could help! Good luck!