Medically discharged from phase 1 training

[Alisha]

Recently my other half joined the army, he got to week 1 of training and pulled in as they found suspected sciatica on his medical notes from 2+ years ago, he has never had it, it was just a pulled muscle, he scored highest in barb test, fastest time running, and physio said he was a strong candidate yet he has been medically discharged. How can he appeal this? He is ringing his Dr Monday to have it changed but he is devastated, he wants to appeal or something to get back in but not sure how, his files being send to a medical board but he hasn't been told how to appeal
Many thanks, alisha