Re: yours mine ours accounts
He might legitimately not know how to budget, or at least fully understand it. Since you have good skill in that, sit down with him, teach him how to do it and devise a plan. Write it all out, put it in a calendar, and encourage him to get anything he can out back on auto pay.
If he is unwilling to do this, or continues to not make payments, let him know you will be selling anything that has your name on the loan because you are not going to ruin your credit because he is lazy bookkeeper.