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How do I explain why debt on debt is bad to my brother?
This is not the case with your brother only. There are many business which run on this premise. It goes till the time all the conditions are in control and get busted when things goes out of way.  You have mentioned the loan amount and not the monthly repayment amount. Even if you say, a new loan will not solve his problem, what are the way out ? Telling things nicely sometime does not work especially when facts are otherwise. Hence you need to make a compete case study which should also consider his capacity to pay. As of now it seems he has debts of around 20 months of his earning, which can be considered high, depending upon the terms of major loan such as car loan and personal loan. A case study is way out. You can explain him with such case study that he should not go for further loans.