Do you mean Grade rather than year? My DS only did some of the grade exams for his instruments - there is no need to do all of them, though you still have to learn the scales etc from the in-between ones.
DS did grades 3,5 and 7 for violin, and is now working on 8. With bass guitar he only did 8, none of the earlier ones. It's not unusual to skip some. It might well still take him 2 years or so to get to the next exam, it's just a question of whether he has to do the grade exams more often, or just carry on and only do some of them.
I would encourage it. I did grade 3 and was prepped to do grade 5 before I moved to secondary school but didn't have time to take the exam. On starting secondary school the teacher said I had to take grade 4 as I wasn't allowed to miss one [hmmm]. Actually completely put me off playing altogether.
KerryM, ds did this. He did grade 3 piano in the summer term of his last year of primary school and then in September started secondary with a new piano teacher who put him in for grade 5 at Easter (two terms after grade 3). I think it's grade 6+ where it seems to get more serious, & I imagine your ds will be fine with the skip.