You must not leave them alone together. People sometimes do say these things when they cannot stand another minute of caring for another person, and they usually do not not mean it - but sometimes they do and sometimes it happens.
Do NOT leave him alone with her, ever, for any amount of time.
You BOTH need respite, not just him. You need at least one day a week where someone will come and give you respite. If that cannot be family then please contact organisations to help.
You could start here:
National Autistic Society: Advice, support groups, and respite care referrals for families with autistic children, 0808 800 4104 or autism.org.uk.
Contact.org: information, emotional support, and local respite options for parents of disabled children, including autism. Helpline: 0808 808 3555 or contact.org.uk.
The Respite Association: Offers funding and guidance for short-term respite breaks to ease carer burden. Visit: therespiteassociation.org.uk.
Or contact your local council children's services and apply for free or funded short breaks/respite
Once you have regular respite set up, you can take a breath and think about next steps.