I would switch to the fluoride tabs if I were you. The others may only have a dosage for little kids and may not be enough to actually work to provide enough fluoride protection. The dentist should have been more specific about this with you.
Before fluoride was in municiple water supplies, people had mouthfuls of cavities and fillings. Mouthfuls. I'm one of them. So is my husband. My kids have NO fillings in their mouths because I gave them fluoride supplements. You will have people trying to fill your head with conspiracy theory rhetoric, showing you crazy internet sites, terrible things about fluoride that have nothing to do with the form that we take for preventing tooth decay. Ignore them. Your child's teeth are important to you. People today don't have to live like we did 50 years ago having a head full of cavities. In our great-great-grandparents and great-grandparents generation, most older people ended up having to have all their teeth pulled and they wore dentures. If people had said NO to all the strides modern dentistry has made since then, we'd STILL be wearing false teeth.
One of my sons had 4 baby teeth with no enamel - not even enough for sealants to stick to. I'm sorry if your son has this same problem with his adult teeth. Thankfully, my son's adult teeth are normal. I kept up with the fluoride until he was 13, took him to the dentist every 6 months faithfully, and had them put sealants on his teeth. Good luck with your son.
Dawn