Ideas for dealing with a non-performing co-founder?

Our team is made up of my co-founder and me, and a couple of contractors. Both my co-founder and I have a technical background. At some point most management and operational issues started to be taken care of by me, and currently I put significantly more effort into the company than the other founder. We both own half of the company and our initial agreement did not set any rules for one of the partners underperforming. How do I deal with the non-performing co-founder? Is there a way to retrieve his half of the shares somehow? If it is too late in this case, how do I stop this happening again?