Cunard QM2 dress code for 2 year old boy Ask A Question Forum

My parents are very nicely taking the whole family on a Cunard transatlantic cruise in a month or so and I just wanted to double check on the dress code for my son.  He is the only child traveling with us and we were planning on taking him to at least a couple of dinners in the main dining room (probably not the black tie though).  He is 2 years old. Should I buy him a jacket or would it be appropriate for him to wear a button down shirt with a sweater vest and nice trousers?  To be honest I'm not sure where to find something like a jacket for him and most of the more formal wear for boys that I have found seems to be of the shirt/sweater vest/trousers variety.  

Any advice/feedback would be very much appreciated.  Thank you!