Probably about ten or eleven. And yes, he would have been introduced formally although I would have thought it would “Master Jones” and not “Mr Jones” for a boy.

If he was the senior male of his household, he would be Mister instead of Master, regardless of his age.

Tom Jones

Regardless of the character’s age, the boy looks anywhere from 12 to 14 years old to me. The actor’s imdb page contains very little information and I couldn’t find his birth day.