What do the variants on the PolyT sequence mean?
I haven't read too far into this, but from what I understand the variant PolyT sequence is the number of thymine bases in a short sequence within one of the introns of the CFTR gene. Introns and exons are both parts of a gene structure, but introns are spliced out of the final transcript before its converted to protein (for more information see this article on RNA splicing).   Studies have shown that the presence of a 5T sequence in this region may increasethe skipping of exon 9 in the CFTR gene (there is a nice picture showing what I mean here).  This exon skipping will alter the final protein product translated from the spliced RNA species.

I could not find a lot of information about the 6T allele that your son has, however, I found one short article about a patient with 6T and F508 in the European Journal of Genetics. It references another paper that talks about the length of an additional repeat sequence of GT bases and its affect on individuals the the 5T variant.