I have a 5 month old male bichon and his coat is changing. From the neck to half way down his body it is short and curly and the rest is unchanged. eg long and fluffy.

Is this his adult coat or as he scratches a lot is he pulling out his own hair

It sounds like his adult coat coming through, make sure you keep him well groomed and get the brush right down to the roots, I had a Bichon when I was little and when we took him for his first hair cut we got sent home again with a slicker brush to groom him out because where we hadnt used the right brush his coat had all matted at the roots as the new coat came through :(

Our Bichon did this too also with scratching.   He was nearly 5 month when he started scratching and still doing it now at 17 months.  I was advised to cut his fluffy hair short by the vet that we took him to and my groomer tidied him up a bit (he was a little uneven to say the least :-) )
His hair is fine now though and he's a great little dog. :-)

What are you feeding him on - this could be the cause of him still scratching?

I have tried different hypo-allergenic foods James Wellbeloved, Nature diet (lamb and rice and chicken and rice) Wafcol salmon and potato but think his problem is more pollen related than food related to be honest.  He has been on Atopica from the vets for nearly a month but its not really made any difference.  He has had steroids about 8 or 9 times but the last time he had them they didn't work and he still scratched :-(  I give him evening primrose oil capsules and have bathed him in Malaseb and Episoothe but not really made any difference.  He isn't exactly scratching constantly and it is mostly his ears that he scratches (and the vet says he can see nothing wrong with them, no infection or anything)

he is probably scratching out his puppy coat, thats what my Irish do :)