That isn't splayed leg
It's a leg turning out at the knee. I hate to say this, but it's really hard to fix this.
Do check to see if the tendon slipped, as that is a little different - you will see that the back of knee looks flat compared to other one. If it is slipped, though, and it's been out for two weeks, it'll be really hard to get it back in and to get it to stay in.
We've been seeing it a lot this year in the emu groups, and we're not sure if this is happening because people are selling chicks from adults that have bad genetics, or bad diets, or are breeding brother/sister pairs, or what, but I've seen much more of it this year than in the past.
This is pretty bad, and I don't think hobbling will fix it. You could try to find a vet to attempt surgery, but even then surgery is very risky on birds and he might die just because of the anesthesia. And if the surgery goes okay, it might still end up turning back out again.
I've got an acquaintance up here who bought eggs and hatched a chick that is having this same issue, and she takes the chick to the vet every week and he does vitamin injections, she keeps the chick hobbled, she doesn't let it run around. And even with all that time and money, there has been no progress and last week the chick slipped the tendon in that leg, which she now also can't get to stay in. This has been going on for upwards of two months.
Edit: My friend had her chick put down on Friday. Even with extensive vet care, it was not enough. It's probably kinder to the baby to consider putting it to sleep
