No credit is easier to fix than bad credit, but both will take some time. The simple answer is get some credit and us it well and fix what's wrong with your boyfriends credit.
Your bank may be a good place to start to get a credit card and start your credit journey. Use it for gas, groceries and things you'd pay cash for now and PAY IT OFF every month. Do not be lured into "low monthly payments" for something you can save up for and truly afford.
Your boyfriends issues must be understood to be corrected, but some steps are to pay everyone on time. Pay down balances on open accounts to 1/3 or less then the limit. Dispute in writing any errors on each report directly to Transunion, Equifax and Experian, the credit reporting companies. Do not close any accounts or pay off any old collections unless your lender tells you to.
Some loan officers can assist with credit repair and this could be very helpful for you. Best of luck.