He should, as he is young, be ok with another young male from another store (I would sex them both yourself to be sure!).

Put them in the bathtub, empty, with the plug in together at first so it is a neutral place (and large enough to get away) and if all is ok then add in some toys and after 20 mins or so put them in the tank. Be sure to keep a close eye on them for the first few days.

If you are worried then I suggest you make a split screen for the tank (wood frame with hardware cloth going diagonally across the tank) and introduce them gradually. This is the method which should be used when introducing older gerbs.

Sorry for assuming you had him in a cage typical mistake tha often occurs on a gerbil board I visit even when the "cage" is a "tank"!!!