I would imagine 40 days in an elbow cast is enough to give anyone 'pain' expecially on the medial epicondyle. Be careful not to overtreat the area if it is already in a state of discomfort. Range is going to be blocked by one of two things; joint, or muscle hyperactivity (or both). You need to see what is the true cause of the restricted range. Mobilisation of the joint through movement (like Mulligan's MWM's) might assist.

Also take a look at the proximal radioulnar joint in forearm pronation and supination and perhaps mobilise that whilst releasing the forearm/wrist/finger flexor component.

Is pain limiting the range or is pain one feature and reduced range a second feature. Maybe a local topic agent might assist reduce the medial epicondyle pain as this bone is rather superficial in that area and so topical agents are likely to penetrate enough into the painful structures.

Remember to check neck and shoulder girdle (1st rib) for dysfunctions as he will have been carrying the arm in the cast for the 40 days as well and so the overall mvt pattern would have been one towards flexion. Good luck and please give us an update as time goes by.