A SSL loan from a local CU would generally be $500-1000 with a monthly payment of $25 or so. That shouldn't put much of a dent in your DTI, especially since he obviously doesn't have any other outstanding debt. So no worries. You just don't want to add new accounts in the final 6 months before apping for a mortgage loan.
Other credit card options are various secured cards, available at US Bank, Wells Fargo or some online providers. Most of them will do a hard pull of your credit, which would put a slight ding (about 5 points) in your score. But adding another card (or two) would probably raise your score about 20 points once they start reporting.
It sounds like he might really benefit from being added as an authorized user to someone else's card with a long clean history. See if a relative of his or yours might be able to do that. I don't think it would help a whole lot in the mortgage process but it could raise his score quickly for now.
Another option for adding a tradeline is to get your rent reported (assuming you have a good history there). There are a few companies that will verify and report your rent, sometimes going back a couple years.
