I'd still be careful about Ca and Ph levels until skeletal growth is completed, which is around 18 months, but given acceptable levels and ratio of Ca and Ph, you could have weened off LBP food to an ALS food as soon as he came home.
Omen was on 100% ALS (Orijen Original Formula) by the time he was 16 weeks (as long as it took to finish the bag of Fromm LBP Gold that I had for him). Jazz was on ALS (also Orijen) by the time she was about 6 months. Neither had any growth, or any other issues whatsoever. Omen is currently about 18 months and 82.5 pounds, Jazz is almost 7 years and around 85 pounds ... both are currently on Victor Hi-Pro Plus.
Omen v Wolfstraum
Jasmine Katie v Doolanshaus