silversong
Male societies get along great. They are extremely easy to sex; generally they don't go more than five minutes without bursting into song, especially when crowded at the pet shop. Even if you cannot hear them, the puffy dance of a singing male is unmistakable. And unlike other finches, each society is almost always so distinct it's impossible to lose it and get another by mistake in the confusion of catching them. Males tend to have flatter heads, compared to a rounded, sometimes almost bubble-shaped head in the female, and also wider beaks, with lower bills that project further back than those of the hens. I've never seen a flat-headed female or a bubble-headed male, but plenty of birds of both genders can sometimes have an intermediate sort of things going on, and likewise you can sometimes get the odd exception which does not conform to the beak thing either - usually a big-beaked female rather than a small-beaked male. If a bird has a small bill and a rounded head, however, it's probably a hen. If it's head is flat and wedge-shaped and its beak large, it's almost certainly male. In my experience, at least - and I have 24 societies (5 hens, 4 cocks, 15 fledgelings, all flying around all over the bird room) right now, so I am a bit experienced in this regard haha.
It's interesting how often egg-binding occurs in caged birds - I feel it has a lot to do with the relative lack of exercise; I have never seen it in any of my aviary birds. I recommend - even if you only have males - to keep a cup of crushed eggshells in the cage; I've never seen a society nibble a cuttlebone (if your hen would not either, that would cause her problem), but they love to crunch and eat these up. Lots of broccoli is good for calcium too (and vitamins).
It's best he get a companion soon, but he will not die if you don't. They are enormously social animals though, and sooner the better for his well-being.
A society would be his first choice. However, you can force a bond eventually with nearly any other bird. I would not recommend a zebra finch, as they can be very, very aggressive. A male silverbill, male spice (this one has a very quiet and basic song though), or male of any of the other various munias would be best, as these species are taxonomically close and can still generally communicate and bond easily. You do not want a bird which is not behaviorally similar - a gouldian, while better than nothing, is a poor companion because by nature it is not a touchy/feely bird whilst a society is. It is this same reason which makes a canary a bad companion - though friendly, it does not tolerate the close contact so vital for the society's social needs.