Criteria would be the ability for him to ... see the remaining balance.
For just the two of you, a shared Google spreadsheet is far and away the simplest path to satisfying this criterion, while dwizum's answer handles the other two.
EDIT: presuming you aren't charging him interest, here's an example spreadsheet.
One option is to talk to a local bank or credit union, and see if your plans would integrate well into their online banking tools. You may be able to kill two birds with one stone - get him on a plan to pay you back, and also get him some exposure to "real" banking tools.
For instance, my credit union has a tool in our online banking platform that lets you ...