小孩子VISA and 预防针

1/ VISA 
Assuming your son was born in the UK after you had been nationalised, he needs to apply for his own passport and visa.

2/ 预防针
Assuming you have asked for what kind of vaccines needed for his trip to China, you need to book an appointment with a nurse in your GP surgery, the nurse will ask you fill in a form including what countries to visit, and for how long,etc. She then would advise a list of vaccines your son needs. It is highly recommended that you do so at least 3 months before the trip because of certain vaccine needs a few months to become effective in the body. Remember to tell the receptionist that you need to see a nurse who is qualified to give travel vaccine advice, not all the nurses can do so.

P.S. I brought my son back to China for 2-3 months when he was 1.5 years old and 2.5 years old without suggested vaccine, and everything was fine.