You laid out the pros and cons neatly and comprehensively. Regrettably, you will have to be the one to balance these pros and cons according to your priorities, resources and constraints and make the decision yourself.
Consider some other alternative, such as your paying someone else such as a college kid to do the job.
If you decide to turn him down, tell him what you just told us
From your experience, The current time estimate is unrealistic. It will take much more time and much more effort than he thinks it will.
You are working on several projects off-hours that suck up all your free time. These projects come with tight deadlines and tight milestones and your only way out is by finishing them, and your finishing them is one thing that's not happening any time soon.
You could add that you are being straight up because it would have been totally unfair to him to tell him that you are going to help him when you are not in a position to do so, and you like him too much and you owe him too much to want to do that to him.