There are pre-assembled sets that can work, but since he is 12 years old and 5ft 1″ and a 9 handicap, and the fact you can build clubs for him, you might consider building him the clubs he needs from our selection of premium components. The link below is to an article and chart Ralph Maltby created years ago that is still applicable today. I believe the length you listed for his 6 iron is an error, but no worries. I think the attached link will be helpful. Use it as a guide, but ultimately use your judgment and your sons feedback to get the proper length that allows him to swing comfortably and make solid contact.
As far as clubs for him, you might consider out TS-2 iron or the STi. Both are outstanding and best sellers and based on his handicap, he should be able to hit them well. As far as woods, the TC Series or the ST Series should work. Not knowing the type of turf he regularly plays makes it a little more difficult on the wedge selection, but our M+Series Wedges have a lot of options and would be worth a look, as well as the TSW wedges. All of these models are designs he could play with for years.
Hope this helps.