As an attorney who almost exclusively represents startups and their founders, there's no question you need a lawyer. You pretty much certainly are legally part

As an attorney who almost exclusively represents startups and their founders, there's no question you need a lawyer. You pretty much certainly are legally partners with your co-founder. The informality of it is irrelevant. Having a short meeting with an attorney before you negotiate this is not a bad idea, but I agree that it's not cost effective to have an attorney negotiate this for you. At this point, he may be amenable to an amicable resolution on terms you would be fine with. The reason to speak with an attorney now is to understand better what resolution you want and how it could impact your business going forward. That depends on a number of factors, including whether you intend to seek outside funding. If he isn't amenable to a resolution that would not hamper the business, then an attorney becomes critical. You need to figure out what leverage you have to force him into a settlement of some sort. That can be complicated. Happy to answer any questions, if it would be helpful.