While he may have filed Schedule C in the past when doing it alone, the two of you have now formed a Partnership. You must file Form 1065.
A partnership is the relationship between two or more people to do trade or business. Each person contributes money, property, labor or skill, and shares in the profits and losses of the business. An unincorporated organization with two or more members is generally classified as a partnership for federal tax purposes if its members carry on a trade, business, financial operation, or venture and divide its profits.
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