Even young budgies not yet fully mature to breed will train their courtship moves and in order to decipher the behaviour, you will have to look at the context in which the foot happens to get there.
Is there any heated singing, kissing, intense up close (beak to beak) head bobbing and food exchange between Sonny and Lady?
If the foot comes during or moments after these displays of affection then you will know he is trying to mate.
The other reason for him to place one foot on the back or even a foot at the chest area during an interaction is to show his dominance over the other bird.
If Sonny is being a bully to your other budgies and they seem visibly bothered by him, then it's best for you to place Sonny on a spare cage.
If he gets along well with Lady, you can house Lady with him and continue to pay attention to their interactions and taking the measures to prevent breeding behaviour.
Be sure to check this link, if you haven't done so already: https://talkbudgies.com/budgie-breedi...want-eggs.html
If despite following the recommendations on the link, you notice that Sonny and Lady are displaying continued and intense breeding behaviour, then you will have to house them separately as well.
This link on flock dynamics may also be of use: https://talkbudgies.com/articles-gene...en-flocks.html