Should I be concerned with cofounders background?

How was it his fault? Your title made it sound as if he had a drug or mental problem.

The best entrepeneurs (and great salespeople who eat and live off commission) ALL have real 'war scars' building income. Its part of being self employed which you will soon learn. It wasnt his fault and he was upfront.

Self employed entrepenuers that had any level of real success had major hits and should be viewed on a different scale than say partnering with a long term corporate type who has a steady income for years with bonuses and company expense accounts and other departments handling income, collection & revenue.

Even banks offer self employed entrepeneurs a higher risk and debt load consideration when extending credit.

What you should be sure of is if his contribution is worth 50% or will you guys need another cofounder.