However much you and mum want to have him back, there is nothing you can do to stop the dementia getting worse. IF he comes back, it may only be for a short time until something else happens that make it impossible for him to stay home, again.
I know how difficult this all is, a number of our family members have ended up in care due to dementia.
Try to think ahead, is it fair for him to yo yo between home and somewhere else, repeatedly? Or is it better for him to be in a care home very near to mum so she can visit whenever she wants, knowing he is safe and well looked after?
You need to first find out where the nearest EMI homes (Elderly Mentally Infirm) are, by looking at the Care Quality Commission website. Then you need to find out how much they charge, how much dad could afford, whether they accept Social Services patients etc. All this can be done before any visits.
I know you don't want to do this. You know that it needs to be done.
If you don't, he may end up like my mum in law, needing an emergency place 15 miles away from home.
If father in law had followed my advice, she could have been half a mile away!