You'll need to start massaging his legs several times a day. When they are warmed up enough, you begin slowly putting them into proper position. You can splint them as well. It's not too late, but you should have started immediately after birth. The legs can still be straightened though.
Google "twisted leg syndrome cats" (I'm new here and not permitted to post links yet) and click on the first result that comes up. It's a very nice article written by show cats online that will instruct you in how to warm the legs, stretch then, and, if necessary, brace them. The good news is that twisted leg syndrome can be treated if it's addressed early enough.