Specsofwhiteallaround
There are some alternatives.
Firstly , IF your DH dropped back to old salary grade you WOULD still be entitled to about £120 a week UC .
I have calculated this on the information supplied ..
Earnings DH £35k
Rent £1k
Council Tax (guess but high rent area around £1980 p. a)
This is £6k alone that you weren't previously claiming .. HOWEVER ..
As you say, DS is now diagnosed. DLA for him (standard award for child who needed portage and speech therapy- but yet a welfare specialist to claim it , they know these forms !)
DLA (or PIP depends where you live)
£83.30 per week. (Low rate mobility mid rate care)
Carers allowance for you. £67.25 a week. (If he is awarded mid rate care)
Disability and carers are NOT Means tested so even if DH stays in his job you will still get those.
But if he does go back to previous grade, you are entitled to £292.05 a week which you are currently not claiming. That's over £15k a year to help you with a disabled child. It will also allow you to stay at home if that is what you choose.
Get the DLA / Pip done whilst DH takes a rest. Then reevaluate.
If you want advice please PM me. I do this for a job.