hi
if the enamel is weak but not cavitated a high fluoride toothpaste or liquid would help remineralize and toughen the enamel.
the amount of time it is in contact with the tooth surface is crucial the longer the better.
so it would be wise to have mouth trays custom made to hold the fluoride against the teeth without it being washed away
cost approx 140 for trays and fluoride can be bought inexpensively in most places