For a parent,this can be one of the most distressful and confusing presentation.As a child psychiatrist specializing in children,I see too often this being a perfectly normal developmentally appropriate behavior that need sensible management.
Imagine the child being confused about this new found feeling and not aware of how to deal with it accordingly.Below steps maybe helpful
1.Child needs to be spoken to re body changes,sexuality and safe behaviour. You can do this or get a professional to do this
2.Rule out potential medical syndromes,a pediatrician or psychiatrist will be able to do this
3.Do not show negative emotions like anger/disgust or use harsh punitive measures.
4.seek other parents of children with intellectually disabled.Often helps.
Dr Ramprasad Attur
it's not a disease it's just a desire...it's ok if he masturbate once or twice...too frequent masturbation may cause him problems....try to avoid frequency by distraction methods like yoga,meditation,listening to music,by doing activities he likes etc....Masturbation is only considered a problem when it inhibits sexual activity with a partner, is done in public, or causes significant distress to the person. It may cause distress if it is done compulsively or interferes with daily life and activities...consult psychiatrists they will do the needful..