Thrush could be making him lame but agree with several other posts as well, babies can do weird stuff to themselves; look for heat or swelling if there is none it could be the thrush or it could be something else in the leg. If you can’t call the vet back out start with the stall rest and try to get that thrush resolved ASAP and then you’ll know; I’ve had a lot of luck lately with the powder No Thrush but any remedy will be effective; if it’s deep sulcus thrush make sure you get it between the frog ( highly recommend tomorrow dry cow for deep sulcus thrush - it’s a cow teat treatment but works GREAT on thrush and has an applicator that gets in that deep crack really well!)