Employment before incorporation

Questions off the top: (i) why a DE c-corp?' and (ii) why wait "a few months" to incorporate?

If you are not incorporated, there is no "company" beyond your de facto partnership, that partnership and its partners' rights and obligation, in the absence of a partnership agreement, being governed by your state's partnership act.

You can have an independent contractor or work-for-hire type agreement in place between the partnership and this "intern," but such an agreement (and the IP assignment provision you'd like to include therein) will need to be supported by some form of consideration. Further, even if you do have him sign on as an IC, your treatment of his internship will need to be qualified by state employment laws, e.g., you can't treat him like an employee, not pay him, not pay certain taxes / overtime, etc. and expect to own what he creates AND not be begging for a wage dispute. Doing the foregoing as a partnership - where the partners may be personally liable for the business' acts and each others' - is asking for even more trouble.

Your question and tacit proposal underscore your need to speak with a local IP attorney in private.