Baby Pigeon can't grip

(uk)

Hello, We found a around 4 day old baby pigeon and have been hand raising it for the past 4 weeks - hes doing really well and is almost ready to fly!

Noticed this morning that he has become (almost over night) to not be able to grip with one of his feet and it looks smaller and thinner than the other almost deformed. He was happily tottering about but now will not walk and when given a little push the foot curls inwards and balance is lost. he also looks lopsided as the bad foot sides wing is now lower

Has he injured himself whilst I am at work? or is this something more serious? how can I help him regain his foot?

Thank you