It is so difficult when your loved one does not see his own needs. Instead of letting him know that you will have someone come in to be with him, I wonder if someone can come over and just visit with both of you. This way you can form a relationship with this visitor and see how they get along. When they are "getting along", you just leave and let the visitor know that he/she is not to leave until you get back. Always leave an emergency number with this visitor and let him/her know what is going on. This may take several visits before you can actually leave.
The Alzheimer Society may provide such a service.
I do this for a friend of mine whose husband has Alzheimer's. She went to see her friends in Florida for 10 days and I would come over but the only way he would let me in was to bring his favourite food (home made bread). I had been there on a weekly basis some weeks before she went to Florida.
Now that my husband is showing similar symptoms we are a good support to each other. I can still leave my husband by himself.
Wishing you well.