Having worked in the field and personally served family and have chronic illnesses myself, I understand a lot of your issues around this.
Here's a quick thought for you. The inital anger will wear off as the routine develops. You have to weigh the benefit for both of you against his anger at a lack of control over things. Typically there is going to be some agitation over something and it might as well be something helpful for you. And him.
Suggestions... have the agency text or email a picture to you so you can show him " your helper" thats coming . Great if you can print it out. Visuals tends to remain in some
individuals minds better then words. Then show it to him as the individual arrives so he can verify its right.
Have shorter times initally , so an hour daily , instead of 4 hrs 2xs a week . Daily habits are easier to become routine.
Emphasize the helper is your helper ..not for him. Until he's a little more accommodating. Then you can shift to longer times and spread out. Usually a week is all it takes.
Have them identified by a specific task and timeframe if possible .
"My friend, Sara, is coming at 11 today to make us lunch ."
" it's 11am, where is Sara? Oh there she is! Hurray! " or " today is bath day. John coming to help with that! I can't do it becsuse he's stronger he can help you"
Something like that
Setting a timer to audibly go off at the time they are arriving can help you set the tone . " oh look , it's time for her to be here now"!
This removes some of the concerns he may develop.
Sometimes just having them stop by to " visit" and see how they can help and leave after 30 mins , with a conversation in front of him " ill be back tomorrow " etc can also ease him into it.
Having him busy with an activity he likes, can also help.
Human nature also, for his age group, may be for him to accept a female helper easier rather then a male. His natural instincts may cause him to shift to protection , albeit confused one, with a unknown male in the home.
Your reaction to his anger, agitation can also feed it or ease it.
If you have apprehensions on how hes going to react, or even guilt for asking for help, fear of how this will work out etc...those emotions translate to him.
You need to have confidence in your decion that this is the best for all of you.
Also, Sometimes a firmer tone on your part or silence to his frustration, works better then tender appeasement.
Remember his mind is not necessarily in his control. Frustration, fear, anxiety can all represent as anger.
Remembering that his emotion will pass. But your need for help will not.
You deserve support in the amazing job you are doing of caring for him. You matter to.
Did the agency have tips for you as well?