I wouldn't be surprised if music is transposed up to grade 5 at some point as there seems to have been an uptake in tenoroons and a new Children's bassoon has also been developed which is half way between a bassoon and tenoroon.
BB played on a tenoroon for about 6 months, although he never practised and messed about on it. He then moved onto a full size (he was 8) and then to a short-reach (only one available) and is now back on a full size (at age 10). So it is perfectly feasible that your son will manage a short-reach in a few years. I'm not a huge fan of the tenoroon as I don't believe it can teach the correct technique for progression onto a bassoon. If I had another child
and they were small I certainly wouldn't consider starting them on bassoon until at least the age of 10. BB is a strapping lad, but on the whole it would be best to focus on the recorder to develop musicianship and then pick up the bassoon later. I know lots of second study 12 year old bassoonists who got to grade 8 easily within two years of picking it up having previously played clarinet or flute.
I should add that it took a good two years for BB to get to grade 3 and then half a year to get to grade 5. His time to grade 6 was a couple of months from grade 5. My point is that it can take a youngster a long time to make more than average progress on the bassoon but once they have developed more, in physical terms, the rate of progress seems to accelerate.