Mother's initial to son

1. No. Legally there is no bar,

2. However, title is universally used for identifying the paternal lineage,

3. Your such step may be questioned by your own son when he grows up with the logic that infant child is not the sole property of mother to arbitrarily try unusal things for self satisfaction, which may affet him later on.

There is no restriction or  bar to use mother's name as initial by the child. Even both the parents names' can be used as initials by the child.  The season for parental lineage is all gone, a child can be recognized by his mother's name too.