If there are no safeguarding concerns for your son, then I don’t see a reason as to why he couldn’t see his son. He wants to be in his son’s life and he should be given the opportunity
Are you hesitant because of you or because of the child. If it’s for the child then protect your kid if it’s just because you are not in good terms let them see each other. You don’t want him resenting you for keeping him from his dad.
If there are no safeguarding concerns for your son, then I don’t see a reason as to why he couldn’t see his son. He wants to be in his son’s life and he should be given the opportunity