There is nothing wrong in taking him if he as you have described. However, you shouldn’t ask him to come with you. If it came from him, take him.
If he asks you to urinate, cut your prayer and don’t take him after that.

Am I allowed to take my 4 years old nephew to the masjid? He does not wear a nappy now, what if he needs the toilet and asks me to take him to the toilet during salah, what is the correct procedure? Do I break my salah etc?