Google MAY be the answer if he is tech and gadget minded.
I use google calendar and google docs, both which can be shared with other people and accessed from smart phone or laptop. I get email alerts for things on the calendar.
I leave the alerts in the inbox until it is dealt with.
So, if I have made an appointment, I note it on my google calendar via my blackberry (£12 a month) there and then.
If I have something important other than a date I use google docs shared with my wife - a sort of 'on the go' notebook. Again accessed on my blackberry or pc.
It is inputting the info there and then that counts. If he can't be relied on to do it straight away it won't work, but at least there are no bits of paper to be lost.
Speak to your GP. Ask them to notify you if he has an appointment. Make sure he is with you when you arrange this so he can give his permission.
Make the GP understand his memory problems - it could be symptomatic of something that can be treated, or needs to be treated.
Take over responsibility for household bills, banking, insurances and so on. The less responsibility he has the better it will be for both of you, but be sure to discuss everything with him so he is still involved.
If he accepts he has a memory problem then he should be appreciative of a method to make reminders and calendar inputs there and then. He will hopefully see the benefit of it.