music grades are they equal in difficulty for different instruments

swannery I agree that for me, and for my DDs, singing a folk song is definitely easier than learning scales. For DS though I know he'd infinitely prefer his grade 5 scales to singing even a grade 1 level folk song. Because he can't sing and hates singing. DD1 has always found her singing exams to be easier and more enjoyable than her other studies - until the compulsory foreign language songs kicked in at either grade 6 or grade 7. I know she found her grade 8 recorder exam infinitely easier than her grade 7 singing, just because of the foreign language lyrics which she found almost impossible to learn (well, she got distinction so obviously not actually impossible - but far more of a worry to her and more effort than the playing). Singing is easy (up to a point) for those who can sing. For those who can't...not so much.

You're forgetting that he's been singing in a church choir for years. So he's been learning singing for all that time. That will have helped his singing skills in lots of ways, not least by improving his tone. Starting singing grades from scratch, without having been in a choir, would be much more tricky. I think that most people find the sight singing part of it the hardest bit, but again your son will have plenty of experience of sight singing from choir. Singing a folk sing has to be easier than learning scales.
IME some instruments are easier than others in terms of passing grades. Piano is notoriously hard at grade 1, whereas with some brass and wind instruments grade 1 hardly seems worth bothering with. The stringed instruments are harder than the easier wind and brass instruments too.

From my experience... which is fairly limited:

Violin ABRSM and violin Trinity look around the same, though of course you can avoid scales. Scottish board fiddle grades look technically way easier... but while remaining in first position up to grade 5, factors like speed, ornamentation, variation of the melody etc. come into play.

Viola ABRSM grade 8 looks around the technical standard of (in my opinion) violin grade 6. That is primarily because the higher positions aren't used so much by violists, while there is likely to be greater emphasis on tonal quality and expression. Having tried to play the grade 8 viola music, they are actually of a similar standard with a lot of double stopping required - it just looks easier because it's not high.

ABRSM classical piano and Rockschool contemporary piano are actually more or less on a par. That is to say, I find the Rockschool grade 8 rather technically challenging. In the early grades, however, the Rockschool pieces are very formulaic and rely heavily on patterns, perhaps because many students will be learning them by ear? There are more scales required for Rockschool if you take the full exam (rather than performance certificate). I haven't seen Trinity but I've been using LCM jazz material. The LCM grade pieces for jazz piano are marginally easier than ABRSM grades, but the jazz grades require improvisational skills even at grade 1 and by grade 6 it's like "here's a lead sheet, play!" They are assessing a totally different subject.

Flute and clarinet grade 1 are way easier to get to than violin grade 1, even though the music may look of a similar standard. I think the clarinet grade 2, however, is a big jump up while violin grade 2 is not a huge step up from grade 1. All the instruments have their big jumps at different places.

Singing is... well... singing. For a start, you'll only ever have one note at a time. You have to sing the pieces from memory. And you will be marked on intonation (not an issue on guitar), breath control/technique, vocal quality and expression. Nerves can seriously affect intonation. But pieces can look easier than they are, and perhaps be easy to play/sing the notes but hard to sing/play really well.

Nationally, all the exam boards seem to be treated the same by Ofqual, if that helps, but they assess slightly different skills, I think.