OP I would follow the advice of the Admissions Tutor - he knows his stuff.
Although they try and organise the hand over between Housemasters, there can still be unforeseen changes within two years.
The house with the 17 MS and MEs that peteneras describes sounds ideal, but the very musical HM who brought about that mix was replaced last September. I know people, with sons in that house who have just finished F block. They all chose the previous HM two years in advance, but by the time their boys started last September they had a completely different HM.
The new man is great, but in a slightly different way. I don't think he shares the same deep love of music, particularly the active participation in playing music with the very gifted musicians in the house, that his predecessor had, and thus the emphasis will change over the next two years.
It might seem that you won't have a good enough choice if you wait, but you will. Boys who have conditional places do decide to go to other schools, either because they have changed their minds or won scholarships to other schools, or they have been successful in the KS exams, so quite a few places in a variety of houses will become available nearer the time.
Finally, remember that your son will have not only his HM but his Tutor as well. The Tutor is as influential (if not more influential in some ways) in the pastoral care as the HM, making sure that boys are coping and happy. There will be a lot more flexibility of choice here, and I'm sure that Eton will make sure that your son has a tutor who will be best suited to his needs.
Very will done to your DS. I hope your bursary application goes smoothly too.