By your post, it appears that your son is interested in buying in Eckington, in DC. While buying or renting close to his work in Arlington may be more convenient, if he really wants to live in Eckington, I would suggest starting there. The home prices would be quite a bit less expensive and the District just recently renewed their first-time homebuyer's credit of up to $5000, which should be taken into consideration when obtaining a mortgage. Also, if he is working in Arlington, the commute from Eckington would be going against traffic, thus making for an easier commute both ways.
When getting your mortgage, I would advise going with a local lender, not a broker. Brokers will find you a lender for a fee, something you can do on your own for free, and that fee gets added into your costs. If your son's only real debt is his student loan, he has good credit and a decent salary, then, in my experience I would say he probably won't need a co-signer. It's also going to depend on the sales price. I have a list of local lenders I can recommend who can answer all of your questions.
The difference in rentals between the two places, Eckington and Arlington, is there are more condo rentals available in Arlington, whereas in Eckington you're looking at house rentals and there aren't nearly as many available. Studio and 1BR rentals in Arlington typically run $1300-$1500. I found a couple of houses in Eckington with 2-3BRs for around $2500 a month, so he'd probably need to get a roommate to help out with rent.
I would be happy to help your son decide which will be his best way to go, rental or purchase, and where he would like to be, DC, Virginia or even Maryland, as I am licensed in all three. We can go see some properties and see which neighborhood he likes best. Feel free to contact me anytime.
Keller Williams McLean