Awe poor guy... I can't give any info on the subject with his injury BUT I can tell you that accidents happen so you can not blame yourself the main thing is your took him to get treatment and are doing everything you can to help him heal
I have had 5 knee surgeries over the years and before the 2nd one I was flat out told I would never run or be able to go to the gym again or be able to bend my knee 100%... I proved the Dr wrong yea I have pain but just push through it so I am sure your little guy will do the same as long as he has a loving family supporting him