You'd be better off noticing the patterns in his misestimation and adjusting his estimates accordingly in your own head.
Better yet, encourage him to become a better estimator. The book Software Estimation: Demystifying a Black Art presents several strategies for developing this skill.
For example: before starting a project, classify it as “small”, “medium”, “large”, “x-large”, etc. (If you don’t understand what you’re supposed to be doing well enough to even make an estimate, you have a bigger problem.) After the project is over, record how long it took. After a few months of work you should be able to see what ranges of time are associated with what intuitive size estimates, and then you can translate your intuition into numbers.
Better yet, encourage him to become a better estimator. The book Software Estimation: Demystifying a Black Art presents several strategies for developing this skill.
For example: before starting a project, classify it as “small”, “medium”, “large”, “x-large”, etc. (If you don’t understand what you’re supposed to be doing well enough to even make an estimate, you have a bigger problem.) After the project is over, record how long it took. After a few months of work you should be able to see what ranges of time are associated with what intuitive size estimates, and then you can translate your intuition into numbers.