Teacher or subject dependent selective behaviour

[Blablaboom]

DS has just started secondary but in his old prep school. He will move to a new school in y9.
Unfortunately, DS seems to select which lessons he behaves/works well in and which he can just ignore. If he finds the teacher boring or if he dislikes the subject, he does not apply himself or sometimes distract others. He's already had several detentions. We've had several meetings with his form tutor. There doesn't seem to be much change. His homework has improved compared to September but in class, it seems to be the same. Likes teacher/subject, no problem. Dislikes the teacher/subject, plays up.
Anyone been there? How do you make the child accept that he's got to respect teachers or subjects in the same way? DS is very strong willed (always has been). The school think we should test for ADD. We think he's just decided to do what he likes. A battle of wills.
Thank you!
(I will ignore all the can't you control your kid trolling type comments)

of course, but the Neurotypical will still manage to do it whereas the ND will really struggle and may display the sort of behaviors the OP is describing

I’m not saying the son isn’t ND but it’s definitely not the case that all NT teen boys can behave in a subject they dislike and are bored in. Many will misbehave and distract others messing about.

Also just FYI OP- there is no
such thing as ADD. There are three subsets of ADHD- inattentive type, hyperactive type and combined type. If the school is using terms like ADD, then they need to update their training.