Sherlock most likely has a bacterial infection of the crop. I say most likely because that is a common reason for vomiting in budgies. BUT.. I will emphasize, as Deborah said above, only a vet who is qualified with birds will be able to properly test and diagnose your little one. There are other things that cause vomiting too. We can only give you our best guess.
Crossing fingers that Sherlock is well again soon. Please let us know what the vet says!