You use the title co-founder, which indicates a partnership and mandates that the tech hire (I'm assuming is the CTO) be provided with the perks and benefits of other co-founders. If you are using the term co-founder loosely and there is no formal co-founder designation in place, then wait 6 months to a year. Why? The tech hire was not part of the original team and equity is ownership. If you give equity it is highly expensive to buyout (think divorce settlement without a pre-nup). If you do give equity to this employee be sure to have a solid attorney draft the agreement.