Speak clearly and with dual eye contact i.e. make sure he is looking at your face. Stop all activity around (no multitasking). Give clear, short instructions and follow with a complement or kindness if it succeeds. For writing at 3.5 years old, I would work with him in very short purposeful sessions again with lots of praise for good work.
He is 3.5 years old--don't expect too much. It takes time, but you have started by wanting to try. Good luck.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics