Prenatal drug exposure effects?

I adopted my half brother at 3 months, my mom lost custody due to drugs. However when I got him I also noticed on his Medical record, drug exposure. To heroin. Now fast forward 4 years he will be 5 in march. He has trouble gaining weight he is only 31 lbs which is the size of my two year old. Dr said he is gaining weight so we shouldn’t be concerned but he is so tiny how can I not be concerned. He also has acid reflux and we have tried changing diet, nexium and Pepcid. Nothing is working and he gotten FIVE cavities in 6 months from the spitting up in his mouth even tho we brush his teeth 3 times a day... what vitamins can I give him to Strengthen his teeth and I have heard pediasures do not work so is there anything he can have to help gain weight? Could these be affects from the drug use? I’m so sad for him and just wanna make it better so he’s healthy and thriving.