Hello Duchess, I'm sorry to hear that your son is injured and is coming home.
When he comes home and heals, if he wishes to re-enlist, I am sure he can do so. (I have heard of other separated recruits returning and completing boot camp.)
He should take it easy on himself (physically and mentally) at first, which will give him some time to research stress fractures and how to avoid them next time -- proper diet, cross-training, weight-bearing exercises, gradual increase in training, etc.
Wishing him all the best!
There's a very good chance he can reenlist. He will have to get waivers but it can be done. Here's the link to the re-enlistment codes. Be sure to look at it.
And remind him he did not fail - it's just a minor setback. Come home, heal and then get with the recruiter again! He's got this!