My son has sat grade 5 violin now and is preparing for grade 6 with the AB. However he has not achieved grade 5 theory yet. He also plays trumpet and has sat grade 6 with Guildhall.

I know that grade 5 jazz is an option instead of grade 5 theory. Can he sit Jazz rumpet grade 5 and then be enter for grade 6 violin? or does the grade 5 jazz have to be in the same instrument?

The regulations say:

Candidates for Practical Grades 6, 7 and 8 must already have passed one of the following qualifications: ABRSM Grade 5 (or above) in Theory, Pracitcal Musicianship or solo Jazz Subject
Trinity Guildhall, LCM, AMEB : Grade 5 (or above) in Theory

Grade 5 Jazz Trumpet then Grade 6 Violin seems perfectly acceptable. They don't do Jazz Violin yet anyway.

Doesn't taking Grade 5 Theory seem likely then?

Personally I think Jazz Grade 5 is so much easier than Grade 5 theory - I don't really understand how it can replace theory, as although some understanding of chord structure is needed it really does not have the depth of musical knowledge needed for grade 5 theory. If I had a pupil desperate to go onto Grade 6 then I would let them do Grade 5 Jazz to meet the requirements, but would insist if they wanted to go on and do more with music, that they did Grade 5 theory.

I think my son who is 10 will take grade 5 theory when he is ready. He enjoys playing more so I thought I would encourage him to take grade 5 jazz trumpet seeing as he is too immature to sit grade 7 trumpet yet.