From duhaime.org, a canadian legal website:
When engagement rings are given before the engagement is broken off, the rule is whoever broke off the engagement forfeits the ring. So if your boyfriend decides that he doesn't want to get married after giving you an engagement ring, you can keep the ring. But if the woman decides to break off the engagement after accepting the ring, she has to return it to her suitor.
Also, if the ring was given at any time such as valentines day, christmas, your birthday, anniversary etc it could be said to be not totally dependent on fulfilling the marriage and the giver doesn't have the rights to have it back even if it was YOU who called off the marriage. And if you called it off FOR GOOD REASON, he also would likely not have the right to receive back the engagement ring. Considering how long you were engaged, I would say he received something approximating marriage in exchange, so unless he can give you back 6 years of marriage-like existence I would say his claim on the ring is weak. However, emotionally you may like to have him just pay out half the cost, do you think it's good karma for your sons to offer this ring to a potential spouse?