When they get that sick and you have already lost one and with your level of experience and the level of expertise of your adviser, it might be a good idea to bring in your veterinarian on this case. You may have more issues going on than what LA200 and probiotics is going to cure.
I would be thinking he missed his vaccinations, ibr, bvd, and such.
Watch the temp, and listen to his breathing, might need to get a little more aggressive on the antibiotics.
The dosage for LA200 is 4.5 ml per 100 lbs body weight. Unless your calf weighs 200 lbs., you need to increase your dosage. Talk to a vet. There are better drugs available than LA200.
Draxin is my first choice Nufloor second Micotill third [ wouldn't use it as it can be fatal to humans but it does work.Consult your vet as they have 8 years of school plus practical experience.The symtoms you give are ones that aren't the first signs you look for. To me a slight droop of ear means treatment.Many cattlemen can tell they are sick before they actually are.