As CR correctly states above, when looking at disability benefits, the starting point for benefits is the disabled individual. In your case that's Child DLA for your son.
Child DLA looks at the 'care' needs (such as getting dressed, bathing, eating, etc) and the 'mobility' needs (physical ability or supervision outdoors) of the disabled person. When then person is a child it can get a bit tricky, as clearly all 5 year olds require a fair amount of care, and so it looks at those needs which are significantly more than other children their age, and also any additional difficulty or support they may need as regards their development.
It's useful if there is evidence from some external body (e.g. doctor/nurse/etc) giving some professional insight, however they are unlikely to be able to go into a lot of detail, as I assume your son is cared for by your wife and yourself and as such you are the ones with the knowledge of how his condition affects him.
There is loads of really good info on the citizens advice website https://www.citizensadvice.org.uk/benefits/sick-or-disabled-people-and-carers/disability-living-allowance/
To claim you can request a claim form from the DWP by calling 0800 121 4600.
If the care component of this is awarded at either middle or high rate then your wife can claim Carer's Allowance.
Hope this helps.