hello Paarl
welcome to TP
I'm guessing that your husband is no longer able to dial 999 on a house phone or mobile and make it clear what would be needed - even if you had a dedicated button he would just have to press once
if he would be able to press an alarm button on a pendant, there are services such as telecare - the person wears an alarm and if it is pressed this is picked up by a call centre who use a speaker system linked to the landline to speak to the person and decide whether another call needs to be made to family and 999 - I had this for my dad and it worked well until he wasn't really aware of what the button was for, or even what an emergency was
to be honest, if your husband isn't able to deal with an emergency, then really it's best not to leave him at home on his own
have you had an assessment of his care needs from your Local Authority Adult Services - he has a right to this, as you have to a carer's assessment - it might be that some support would be suggested eg home care visits, a sitter/befriender, time at a day centre and respite - and I got the telecare system through dad's LA as they will also look at any aids that may help in your home
best wishes