Despite these challenges, I claim success. This has been worn quite a few times already.
Sirdar's Baby Bamboo too, book 346, which despite the title and the beautiful infant on the front ,includes patterns up to a generous size 7 years.
I did intend to make the sleeves to fit when the cuff was folded back, in an attempt to gain some longevity from the jumper, but the yarn relaxed a little with wear, so I am hoping some growth comes along to correct this minor fitting issue.
I used the Baby Bamboo yarn for the jumper, this being rated completely non itchy by the boy, and coming in a wide variety of colours. This one is 150, Denim Dungaree, which sounds suitably boyish. The last colour I used for him was called "Coo", which is not something I mentioned to him when we were discussing colour options.
The yarn is tried and true at our house, being just warm enough for Queensland Spring and Autumn evenings, and machine washable (on delicate wool wash).