Hello and welcome aboard.
Typically we switch from Prozinc to a similar dose of Lantus, but I wouldn't recommend you do that without seeing a spreadsheet first. From today's numbers, he could also be bouncing from whatever numbers he got into last night on Prozinc. Also, Lantus is a depot insulin, it can take a few days before the depot is full and you see the full potential of a dose.
Since you did reduce the dose so much, I strongly recommend regular testing for ketones.