Lots of different replies and advice. It is good to get all the information you can.
If I remember correctly, the op's child can color. How well? Can he stay in the lines? Can he draw pictures that are recognizable? If he can, then I would have to say that he doesn't have fine motor problems and is just stubborn!
Kids are different. Not one child is going to develop exactly like another child. Some children will color at 2, others will color at 6. Their is no mold for children. There are standards that people come up with that they hope will fit for most children, but honestly, the standards don't measure up.
I taught public school for years and in a classroom of 25 10 year olds, I would say their was such a difference in abilities. Putting aside the few children who were labeled learning disabled or gifted, even the rest of the average kids had a wide range of abilities. I had students who had beautiful handwriting, some with horrible handwriting, some who could draw masterpieces and some whose stick figures didn't even look like people!
As I mentioned in a previous post, by dd6 doesn't care to write. I think it is because she is a prefectionist. If she can't write a perfect lower case e, she doesn't want to write a lower case e at all. In fact, last week she finally wrote an e she likes and I don't have to remind her to do a lowercase instead of a capital! It just happened.
OP, the shaving cream idea is fun, also pudding and the pudding is tasty! Also, how about an easel. He can use sidewalk chalk to write on an easel. Don't make him do too much at a time. Maybe just his name once. And don't stress. He will do it in his own time. And, as a side note, many boys that I have taught in the past are not interested in handwriting. They have bettter things to do!
