Note: I've been a freelance designer for more than 20 years.
For design I really don't think I'd do any retainer whatsoever. Creating 5 logos, or 5 sales booklets in a month is worth way more than creating 5 brochures, or 5 fliers, or 5 landing pages. Even if they all ultimately equal the same actual "work hours". One fee to cover hours would simply be cutting yourself short. Ideation and conceptualization has EXTREME VALUE and pricing that the same as general design work isn't doing the designer any favors. Per project pricing or value-based pricing is always a better option for design in my opinion. As a designer, I detest working "per hour" most of the time for anything other than updates, edits, or general maintenance - essentially, "grunt labor" is hourly... creation and conceptualization is never hourly for me.
That being posted, if it's something you are considering.....
I'd say minimum monthly fee, then hourly above X hours.
So, He pays a monthly flat fee. This covers X hours of work each month. Any month requiring more than X hours of work will be billed $XX/hourly above the initial X hours.
So, using random numbers....
$1500 monthly for up to 25 hours of work. (calculates to $60/hr. So gives initial price break)
Anything above 25 hours is billed at $85 per hour.
This provides a flat fee and a fallback should there be more work than anticipated. It also provides a point where you can let the client know they are reaching the "cap" in a given month and anything more that month will be at the increased hourly rate. Which actually helps the client keep an eye on their spending.
The problem becomes setting that "monthly x hours" initially, because you may not know how much work will be involved and the client can't possibly predict that for you. So it may be best to work on a strictly hourly rate for a month or two until you can get a "ballpark" idea of how many hours each month the client may require.
Now.... the client may not like this structure. The client may want one fee to cover everything you may do each month, no matter how much there may be. That is a completely untenable solution for any designer in my opinion.
One project may take 1 hour, another 50 hours, and another 5 hours, and another 30 hours. You can't cover all projects with one flat fee and be a profitable business, unless you calculate enough hours that ANY possible work could not ever come close to reaching those hours in a month, i.e. 1500 hours or something ridiculously high. This becomes even more untenable when you factor in conception and ideation... that's the bread and butter for a designer and where costs can't always be tangibly calculated. If you create 1 concept one month then 5 the next, they all can't be priced under a blanket fee. That additional ideation has extreme value.
The client may also ask "What happens if you don't have 25 hours of work in a month? Do I get a rebate?"
The monthly fee should be seen as a "retainer" to ensure you are on call and the client has priority at all times. It's a flat fee to ensure you are always available to the client and his/her projects are scheduled first. There would never be a rebate for less than the X hours/month. Anything remaining at the end of the month is to cover your "on call" status. Some months that may be 0, other months it may be a couple hundred dollars due to less work. That's the benefit for you in providing the lower rate for X hours initially. The benefit to the client is the lower pricing and immediate attention. Both parties gain something via the retainer.
If the client doesn't grasp any "remainder is your benefit", then just opt for straight hourly invoicing.