Though I have not really uses eimacs, I have heard that their logic courses are good.
For learning math for fun, we used the AOPS Intro to Counting and Probability and the Into to Number Theory books where my child picked up the books and worked through them on his own time and out of his own interest.
I don't think that there is one online course that can provide enrichment for multiple areas of math. There are books that you can provide for various topics of interest, Great Courses Video lectures, youtube channels etc and let your child pursue what interests him.
