If the allele is auto. rec. then the boy's genotype is a//a.
Each parent must contribute a/ to the boy. Since neither parent shows
the trait, both parents must have "A" also. So, both parents of this
boy must be A//a (i.e. carriers). So far, this works.
Finally, can we then get this daughter from such parents? She doesn't
show the trait, which means she has at least one "A." Therefore,
yes, it is possible to get this daughter from those parents. Conclusion:
the allele of interest could be autosomal recessive. There's
nothing in the available information that enables us to rule out this possibility.