There is nothing wrong with what your son is doing!
He is doing all the right things at the right time: he is gentle so his uke will train with him again. Gentleness might be because your son does not want to feel like he is acting like a bully.
His moves are fine so that he is learning to do them reflexively. Remember repetition makes permanence. This means he is learning correct forms. Speed and power can come later when he grows up a little.
Now, as the next stage of learning, he needs to do the techniques faster and with more power. Those thing will happen naturally as he trains more and more. As his uke (and himself) becomes more skilled in ukemi, he should see that doing a technique powerfully and fast does not mean harming his uke. Neither will tori harm him when the roles are reversed. My guess is this last paragraph is what you were looking for.
You have a caring son with empathy for his fellow human beings. This is a good thing™ and you should be proud of it.
Do you live in a war zone where you expect your kid to fight? Are there no figure of authority around that can deal with bulling in a civilised way? Or do you want your son to use violence to solve his problems? … Of course you do not. I was exaggerating on purpose.
If you are concerned about self defence, then you should look into kid self defence, as martial arts are just but a tiny part of it.