Yes, it can be assumed that the phenotypic difference is due to the two alleles of a single gene, which can be explained in following manner
a. Sons inherit X chromosome from his maternal parent (?) and Y- chromosome from paternal parent (?). The traits of earlobes are dominant X-linked and can be expressed only when both traits are in homozygous condition. If the mother is heterozygous for earlobes, the son may have attached ear lobes while the mother having free earlobes. The son might have received recessive allele. Hence, the character is not expressed.
b. No, it is not possible to decide whether the allele for attached ear lobes is dominant or recessive. This is because the provided information is limited.
If attached ear lobe is dominant trait, then the mother must be homozygous recessive and the father must be homozygous dominant or heterozygous. Then, there is a probability for the son to receive a recessive gene from the mother and the dominant gene from his father. In case of father is heterozygous, there is 50% probability of having attached ear lobes in the son.
If the attached ear lobe is a recessive trait, the mother should be heterozygous to donate a recessive gene and the father must be heterozygous to donate their son a recessive gene to have the expression of attached ear lobe. If the trait is autosomal recessive or X- linked recessive, the mother should be heterozygous for that trait and the son must receive a recessive gene.