I agree, all kids develop different areas at different speeds, and he sounds fine but if you are still concerned you can ask your nurse to refer your son to an OT, although he may be too young ( i am an OT but haven't worked with paeds). Encourage him to play with things that develop his upper limb and hand muscles (drawing on blackboards, playdough, etc) and I'm sure it'll all happen in time. I find they seem to have "clusters" where they suddenly pick up on and consolidate a lot of skills all at once...then nothing much happens for a while.

BTW: I have a 4.5 yr old who I basically have to bribe to get him to draw - he just IS NOT interested at all!! But talked from very early age. And I have a 23 month old who doesn't have all that many words but loves to draw!! They can vary alot and still be within "normal" limits.

Try not to worry I'm sure he's fine!!