I'm weaning a ten-month old TWH colt from his five-year old TWH mother: is my plan going to traumatize them ?

You could try that, but I doubt it would really work well. The best thing to do is move the colt to a friend's house or perhaps board him for a month at another barn. Then you should be able to put the two back together. It's harder to wean (and more stressful) if the mother and baby can hear each other (they'll call back and forth and be distressed).