I think that once a child has something that pops out of place once, it can happen again fairly easily until it heals. Hopefully it keeps popping out of place because it's still healing, not for any other reason.
Does he have low muscle tone as well as loose joints? The only thing that I know of that would cause frequent dislocations like that is something called Ehlers Danlos syndrome. I'm sure that there are other causes but it may be worth looking into for you.
Some of the symptoms include:
* Joint dislocation
* Joint pain
* Increased joint mobility, joints popping, early arthritis
* Flat feet
* Easily damaged, bruised, and stretchy skin
* Very soft and velvety skin
* Easy scarring and poor wound healing
* Premature rupture of membranes at birth
* Vision problems
I would get in touch w/your pediatrician (if you haven't already) and see if they are able to help figure this out.
The other thing that has been mentioned here is nurses (or nursemaids) elbow but that seems less likely if your child has low tone (hypotonia) or loose joints (hypermobility).