Ways to teach our autistic kids about the natural world

Teaching anything to my autistic kids can be a challenge.  Even when it's an area that they are interested in it can be challenging to support them to learn when they have such significant difficulties with communication.  David is in Yr 7 according to his age but in many ways he's operating like a child who is in Yr1.  He speaks in only a few words and needs to be shown most things to learnt them as he only understands some of what is said.