this is what is called perching, when the bird is comfortable, tired and wants to sleep the bird will use 1 leg , all birds perch like this and is nothing to worry about what so ever,
if the foot was injured in most leg/foot injury cases the bird will still try and use it and try and stand on it and you would then see the damage, in most leg injuries the leg either hangs low or wonky and a swelling can usually be seen....
how long have you had your bird for, if you havnt had him for long or only a matter of weeks then perhaps he just hasnt been comfortable enough to perch properly infront of you, takes a while for them to get used to their surroundings,
here is a link to a google search page with lots of pictures of birds on 1 leg, this will help to give you an example of what i mean, even before the bird tucks his head under the wing it will still stand their on 1 leg before it goes to sleep properly, it is just a sign of a well adjusted bird that is comfortable in the surroundings
bird sleeping? - Google Image Search
good luck with him
katie