This is a very rough question. I was in a situation like this a number of years back that lost money. So, in the end, the question wasn't who had what share, but who actually had the money available to get rid of the project.
What I would do is put a value on your credit. Let's say that's 20k or something. Then he puts that amount in, and you do everything from there on 50/50. I think we'd need to know about how much money is going to be needed up front before advising on the split.
Here is some advice based on my experience. Your partnership agreement has to have all the duties lined out, not just where the money is coming from. For example, who is going to find the tenant? Who is going to get insurance? who is going to pay when there is no income? Many questions. Also, who is going to be on title? Who's going to take the tax benefits?
One idea that could be helpful, would be to have a reserve fund. For example, give your brother a 70% share of the property, but only if he establishes a significant reserve fund available if anything goes wrong.
Another idea would be to do a limited partnership. You're the general partner. Establish a $ amount for your brother to invest, calculate whatever return he would get, and what circumstances would prevent that return, and then do everything yourself. This would be a little cleaner, and then everything is your responsibility and you won't have any disagreements.