First of all, you're a good parent for even joining this site and asking for help! Don't ever let anyone make you feel otherwise. I think we all go through the "judgemental" looks from others around us who don't even understand what we go through at home.
You and I are similar in that we had adopted our nephew who was exposed to meth inutero and alcohol.
Dental hygiene was always a problem and our son, 6 yrs old has to be medicated for any dental treatment.
Good luck with the hygiene routines! I haven't reached that age level yet for my boy but I swear the medications help with even tooth brushing now.