I think some elements require the ability to understand. One can receive Holy Communion only if he or she can discern the body and blood of Christ - “to discern the Eucharistic from ordinary bread, to realize in some measure the dignity and excellence of the Sacrament of the Altar, to believe in the Real Presence, and adore Christ under the sacramental veils”.
Thus the Eucharist, in which Our Lord is truly present, can only be given to those who “can distinguish the body of Christ from ordinary food and receive communion reverently”.
Pastors are encouraged to consult with parents, those who take the place of parents, diocesan personnel involved with disability issues, psychologists, religious educators, and other experts in making their judgment. If it is determined that a parishioner who is disabled is not ready to receive the sacrament, great care is to be taken in explaining the reasons for this decision. Cases of doubt should be resolved in favor of the right of the baptized person to receive the sacrament.
Regarding Confirmation, it is available to “all baptized, unconfirmed Catholics who possess the use of reason …] if they are suitably instructed, properly disposed and able to renew their baptismal promises”. However: “Persons who because of developmental or mental disabilities may never attain the use of reason are to be encouraged either directly or, if necessary, through their parents or guardian, to receive the sacrament of confirmation at the appropriate time.”
Confession is different: “Only those who have the use of reason are capable of committing serious sin. Nevertheless, even young children and persons with mental disabilities often are conscious of committing acts that are sinful to some degree and may experience a sense of guilt and sorrow. As long as the individual is capable of having a sense of contrition for having committed sin, even if he or she cannot describe the sin precisely in words, the person may receive sacramental absolution.”
Here you can read more about the position of the Church.
In short: there is a possibility that the priest, who is not perfect and all-knowing, could have done more. I am certain that if she pursues the matter she will be able to have her son receive the Sacrament of Confirmation and work with experts to discern whether he can discern the Eucharist or not.