If the intent is to simply lay out a set of terms for a gentleman's agreement or an MoU (e.g. that you will make best effort to be available for the required hours each week and provide him at least x amount of notice if that's going to change and that he will give you x amount of notice if their need for you is going to change), then it's going to be very easy to construct that. Probably the safest way, and the one that would lead to the least amount of possibility of misunderstanding would be for you both to;
- Sit down at a table.
- Express verbally what you both want.
- Check that you're both happy with what's being proposed
- Share the minutes of the meeting via email afterward.
Since one of the key elements of a contract is that both sides identify and recognise that it's binding, adding something to the notes like "This agreement is binding in honour only" or "This memorandum is not intended to, and does not create any contractual rights between these parties" should be more than sufficient to protect both of you from the ability to sue the other for a breach, for example if you break your wrists and can't use your arms for the next six months or if they decide not to continue using you at short notice because they've discovered a hole in their finances.
You do, however, absolutely want to make sure that you still have a (legally binding) contract in place to ensure that he pays you for any work that you do.