I had this as a baby, but it wasn't picked up until I was 17 months old, and has caused me lifelong problems ever since. So it's great that they've picked it up now as there is plenty of time to fix it.
The STEPS charity is absolutely brilliant at providing support, and they have online forums which contain loads of brilliant tips for coping with casts etc. so check out their website if you haven't already.
If you can afford private treatment, John O'Hara in Birmingham is one of the best paediatric hip surgeons in the country/world (he fixed me where other doctors failed). He was on the team when I was little and he's my main surgeon now I'm an adult.
Also, please remember that whatever your son goes through, it will be way harder than you than it is on him (the only bits he will remember will probably be happy memories, honestly), so make sure you have lots of support yourself.
Also, for him and any subsequent children you have, make sure you avoid things like Jumperoos, baby walkers, Baby Bjorn carriers, trampolines and swaddling as they can all exacerbate hip weaknesses. There's more info on the Steps website and the International Hip Dysplasia Institute websites. Traditional baby wearing can be good for helping hip sockets to develop properly.