"Radioactive Paste" and "Mandalorian Iron" are probably the 2 mid cost items that hold their value pretty well right now.
The best thing to get to really extend your credit cap is "Molecular Stabilizer" but that can go for around 200,000 credits, which is all his money.
Once he hits 50, he'll be able to accumulate 200,000 credits pretty quickly and he'll probably want to just use a stack of "Molecular Stabilizer" if he wants to continue playing. These are not only good for trading to other players, but they are also the most expensive ingredient to make the really good stuff for his own character.
The main issue is that he'll have difficulty making use of those items if he doesn't upgrade to preferred. Here's how you'd go about using your credits after level 50:
You find a crafter that can make the best lightsaber hilt (or blaster barrel) in the game and work out a trade where you supply him all the materials and a tip and he crafts it for you. These currently take 8 "Molecular Stabilizers" and a bunch of other much lower cost items. The total cost in raw materials comes to around 2 million credits, I think. You'd probably offer that crafter another "Molecular Stabilizer" as a tip for his services.
The best way to really facilitate this sort of arrangement is to secure trade with the person, which you can't do as a free-to-play account, but you can do as preferred.
For that reason, if he is enjoying the game and thinks he'd like to continue to use the economy, I strongly recommend trying to convince him to spend $6 and buy something from the cartel market so that he gets upgraded to preferred. If he were to buy an additional crew skill, then he'd have a total of 3 (+1 for being preferred and +1 because he bought it) in addition to the other perks of being preferred (more action bars, more gtn sales, the ability to trade with other players, cargo hold inventory storage, general chat access in all areas). That will facilitate him making money faster as well as making use of that money more easily later on.
This. Preferred never goes away. It's a solid investment + he can grab some nice moddable stuff off the CM.