I too, have a DD, who is the same - she is incredibly bad at maths but a very able reader. I too, found it out in Y1 at a parents evening. I was astonished. No one ever raised the issue with me.
Now, my DD is in Y3, well ahead of her peers in terms of reading, but unfortunately, I had to come to peace with the fact that she is vary bad at maths. When it comes to it, her mind just wonders off, there is no real motivation to solve any problems etc. It is her trait and nothing will change it. So I do 5-10 min maths work with her nearly every day, and that is about it. We don't neglect it, but we don't make a big fuss about it either. That's who she is.
On the other hand, your son is very young, so you just wait and see.
Also, when it comes to behavioural issues, I would think, that he is fidgety because lack of physical exercise and free play. One of my DSs is like that, messing about quite a bit.
Free play and physical exercise - nothing beats that. More homework at home will not help.
Also, I would never tell him to get good to get to the top of the ladder for a reward. He should behave because it is much nicer than not behaving. However, I suspect words won't really change the fact, that he would need far more free running around.