My son likes the books also, but I wonder if after reading a LoF book, for example Beginning Algebra, a gifted student should proceed to the next book in the series (Advanced Algebra) or cover Algebra I with a more traditional textbook.

Your son may also like "Algebra, the Easy Way" (and the trig and calculus books) by Downing. This book, like LoF, uses a narrative format.

"To see what is in front of one's nose needs a constant struggle." - George Orwell