It's unlikely that a colt foal will try to mount his mother when she is pregnant.
Why This Is Unusual
Foals are too young and inexperienced: At that age, they're focused on nursing and learning about their surroundings. They haven't developed the strong sexual urges that lead to mounting behavior.
A pregnant mare is physically uncomfortable: Her swollen belly and limited movement make mounting difficult and potentially painful.
The mare's hormones: A pregnant mare's changing hormones may make her unreceptive to sexual advances.
While a colt might show exploratory mounting behaviors, it's highly unusual for them to attempt it with their mother, especially while she's pregnant.
Important note: If you observe unusual or aggressive behavior in a foal or mare, consult a veterinarian or experienced horse professional.