Hi Simon,
Welcome to the Forum! This is such a beautiful portrait and I can imagine what you went through to get a good photo reference for the boy. His expression is perfect and seems to reveal something very personal and meaningful about him.
What is his relationship with the dog in real life? It's true, something has to be done about his hand. Allan's suggestion might work. I would not make him do anything that is not natural to him. You might be interested in taking a look at the website of Nancy Bea Miller, an artist who includes children with disabilities in her paintings. Her autistic son often appears in her work. Her website is:
www.nancybeamiller.com
She is a member of the Forum.
I agree with Ilaria about the bottom of the painting. Right now both boy and dog appear to be sinking into the leaves. Finishing the bottom of the figures loosely might be a good solution. You have such a confident, relaxed way of brushing on the paint and the colors are subtle but glowing, with no trace of muddiness.
Alex