It's great that you are spending time exploring art with your child!
I have a similar challenge with my child, in that I'm not good at drawing images either. Here are two tricks I used when my daughter was five and used to ask for my help with drawing:
1.) Tracing paper -- cut images out of magazines and then encourage your child to use the tracing paper to copy that image on a blank page.
2.) When out of tracing paper, use magazines, cereal boxes, photos and even downloaded images off the Internet to use as an example. Select images that are made of simple shapes and aren't too complex or difficult to copy. Then help your child break the image down (a curve here, a straight line here, etc. until he has drawn something similar that pleases him). Start with pencil first if your child is a budding perfectionist (like mine), and be supportive regardless of how well the drawing turns out.
Here are some resources on Education.com that you may also find helpful...
It is very good that your child likes to drawing which is a very good sign for a child if he wants to draw. If a child has interest in drawing then you just needs to push him a little by giving him some drawing material. You can give him books which teach how to draw certain thing (especially basic shapes and drawings). I have another idea also which is just the right thing you need. You can check a website www.studyvillage.com which can solve your problem. It offers all the printable drawing worksheet materials, for the beginners or for the school students of various grades. You can print the drawing worksheets and coloring pages and give them to your child. Here I have given some links from the website, you can also check the whole website according to your need. I am sure this will certainly help you.