A lot of birds like it if you put a little mirror in the cage with them.  They look it the mirror and see themselves and they think that they have company.  That way the bird gets cheered up and you do not have to worry about another bird hurting him!

 Thanks for the advice but she already has a mirror but she isn't fooled by it.

 Well first i would suggest taking your parakeet to a vet, because the reason for it seeming 'lost' may not be from a lack of companionship, but an illness. But if it is true about your parakeet seeming sad, then yes you should deff. Get him a friend. Just make sure that the cage is big enough for both. I suggest getting him a girl maybe around his age, but then again any age is fine as long as they're above 1 year/sexual maturity. Hope this helps.