Slipped tendon. It is usually caused by poor nutrition such as feeding turkey poults chicken starter. The tendency to this condition can also be inherited.
It will have to have the tendon manipulated back into place and then secured there. It needs to be fed a high protein turkey or gamebird starter that has the higher levels of protein, lysine, methionine and niacin that poults need.
You can do a search in the Emergencies forum for slipped tendon. Other than the dietary needs of turkeys, the treatment is the same as it is for chickens.
As the poult gains weight, the leg is going to continue to get worse unless properly treated immediately.
You can supplement the niacin and thiamine by dissolving a 1/2 tablet of vitamin B complex in one gallon of water. It needs to be the only source of water and should be made fresh daily.