Despite cultural or parental differences, you are the Mom.
If you think your son has a problem with motor-skills/feeding himself, then ask your Pediatrician. Let your Pediatrician, have the last say.
Kids this age, are not fully coordinated per fine motor skills. Kids are messy when they eat. But mostly, they are learning to be experts at it.
Fine.
Per "eating"... any human should not be forced to eat. I don't know if your Mom is doing this, but a person should eat per their body's cues. ie: hungry or full. If a child never learns this and about their body's cues... then... eating may become a problem... ie: overeating just to please other people, eating to ease boredom or to get emotional satisfaction. This is, dysfunctional eating. And it is a "habit" that can affect a person, even as adults. "How" a child is taught to eat, may then become how they eat. Later as adults. For emotional reasons.
Being small, is also per genetics. Not just because of how a child eats or not.
A child does not have to be chubby or big... if your Momis old-style in thinking and therefore, she keeps feeding your child, because he is "small."
You are the Mom. You decide what is best for your child. And in what he is taught or not.
Certainly, a child this age should be, getting used to using utensils themselves, to eat. If your child were in Preschool, he would have to feed himself. For example. At this age. But again, kids this age are not experts, at being fully coordinated. But they can feed themselves.
Or, you tell your Mom, that the DOCTOR, said, he can feed himself and that he CAN "practice" eating by himself. And that is developmentally, fine and normal.